See section " 3n. When things don't work: High Voltage Section Problems " at the link below. Actually, you should read all the tech docs. These not only answer your questions, but there are also some upgrades that should be performed to these boards.
http://techniek.flipperwinkel.nl/ballyss/rep/index3.htm

You might have missed this in the text from the link I posted earlier:
If your high voltage section is showing really high voltage (like 230+ volts), often the 2N3584 and the 2N3440's have failed. The 2N3440's are easy to get. So what can be used to replace the hard to get 2N3584?

There is a replacement transistor, the BUX84. This transistor has a TO-220 style case (instead of a TO-66 case). You may need to enlarge the heat sink's base/emitter holes to make sure there is plenty of clearance for the new BUX84's base and emitter leads. You can also cut off the BUX84's collector (center) lead. You can do this because the BUX84's metal tab, which bolts to the circuit board, will provide the transistor's collector connection. Remember to also insulate the heat sink from the transistor's (collector) metal tab. Use a nylon washer between the transistor's metal tab and the heat sink. Also use a small piece of heat shrink tubing to go over the bolt that holds the transistor to the heat sink.

If you don't want to go through the hassle of repairing the HV, you can always replace the displays with LED aftermarket replacements. Then you don't need the HV section on the supply at all.

The fuse on the SD board is not correct, and yes, that would cause your displays to not work if they are not getting 190vDC.

You should be replacing or rebuilding the SD board. You need to replace the filter caps if they are original and modify the fuse holder to use a standard size fuse. Replacement SD boards are common. You can go that route if you prefer.

The 230 volts you are seeing is before the regulator, which should adjust between 170VDC and 190VDC. Ideally you want to adjust this closer to 170VDC to prolong the life of the displays. If 230 was actually getting there, that's likely to burn out the displays quite quickly, but you said there was nothing at the HV test points on the displays, that's probably because the regulator section is kaputz. If your displays were getting overvoltaged for too long, they may be outgassed already. You might want to post some clear pictures of those displays, because if we can reasonably determine this you might want to make a different choice as to how to go about getting the game going. Original working gas displays are harder to come by, and often quite expensive.

I figured I would throw my experience with a Space Invaders I'm currently rebuilding in here as well.
When I first got mine and started testing things, my HV sections of the SDB did not work correct at all. Only one display did anything, and it didn't do it right.
I had to replace the Caps and the pot trimmer along with the headers on the board and repin connectors in the harness to get mine right.
Specifically I replaced C26, R35, RT1, Fuse clips and fuse, pin headers and re-pinned connectors.
I'm still using the original board, but with those components replaced.
It still has all 5 of the original displays in it and none are out-gassed or burned (surprisingly).
Once working, I was able to turn the pot trimmer down and got my displays stable at around 165v measured from TP2.
I'll also note that I upgraded some of the resistors on my display boards (the one attached to each display), from 1/4w to 1/2w and re-flowed the solder on most points and replaced the headers. Pretty sure I had a bad decoder chip on my Player 1 display as well that I socketed and replaced. The resistors were replaced as the old ones were Very black and a higher wattage should help with heat issues.
Sorry if that is a ton of information, but wanted to include anything I did in case you needed it.
Good Luck!

^ This is all good info. Typically I find it justifiable to repair the HV and maybe replace a display if only one is outgassed. Then you can get your original display set up and running for the cost of a handful of parts, at often under a $100. When you have to replace two or more displays and repair the HV, you get up near the cost of a digital display set. With a digital display set, you don't have to worry about another display outgassing in the future, and you don't have to repair the HV on the SDB. It just makes good economic sense for the long run. That's why I suggested looking at or testing the displays themselves first...so if you decided to go the route of digital displays you didn't waste time troubleshooting and replacing parts on the HV.

I agree with this.
Going with some led displays over the old plasma gas ones will definitely help reduce issues on the SDB board (no HV section to worry about) and ease up on the rectifier board a bit at the same time. Finding used original displays does seem to be getting a bit difficult. Well, assuming you don't want displays that are burnt our starting to outgas. That being said, they are definitely still out there.
I looked at led displays for my game, but my displays worked (tested them all in another game) and I honestly just love the look of the originals. I've seen some of the led displays get close to the color and glow of an original, but I've yet to see one get it dead on.
If you aren't concerned about keeping it original or keeping the plasma displays, you could replace them with the new digital led displays fairly cheaply and help future proof your game a bit.

Well you have cold solder joins for sure that need to be fixed. That is from the first picture you posted of the bottom of a display board. Top right looks like it needs a reflow to me. I would do the same check on every board you have in there to be honest.
Cold_solder (resized).png

Both of those capacitors need to be replaced for sure. Those both look original. The fuse clips look ok, but I would replace those too and drop a new fuse in.
SDB_Updates (resized).png

These are the resistors i was talking about (I'm pretty sure at least, I would need to double check later to be sure, but you can look it up as well, its a common thing mentioned to do to these boards) and yours don't look too bad. They are a little darker but not horribly so, so you'd probably be fine for now. Mine were boarder-line black and had shaded the board around them a few shades darker as well from the heat.
Display_board_resistor (resized).png

Beyond what I mentioned above, you probably will want to replace the headers on the Display boards and the SDB along with repinning the connectors. Remember to use the trifurcon connectors (not sure I spelled that right) when you can as they are the better replacement. All the parts to do that will not cost much as they are all fairly standard items. You can reference the rebuild guide that @vid1900 made for the SDB as well https://pinside.com/pinball/forum/topic/vids-guide-quick-bally-driver-board-repair-bulletproofing
That is the guide I followed when I rebuild my SDB board and it has worked perfectly since then.

If you rebuild the HV section on your SDB, do not set the voltage below 170volts. Yea, the displays will last longer, but you'll blow out the HV section again. Below 170v puts too much stress on the transistors in the HV section.

looking at those pictures again, your header pins actually look pretty corroded. That could definitely cause issues.
display_headers (resized).png
I'd replace them, several sites sell the board parts, GPE (https://www.greatplainselectronics.com/default.asp) and BigDaddy (http://www.bigdaddy-enterprises.com) have kits you can purchase as well that will have all the parts or you can order individually.

Do not send the header ouns, this removes the plating. They need to be replaced. Your going to need to repin the female connectors on the cable harnesses regardless if you use original displays or upgrade to digital LED displays.

The high voltage goes straight to the displays. Digital LED displays run off the 5V logic power already present in the harness connectors, this construction ignores the high voltage pins in the connectors. They are plug-n-play.

Agreed, sanding those header pins is not a good idea. Once the plating is gone then they are done. Replace the wiring harness connector pins no matter what as they will be needed irrelevant of what displays you use.
As far as I know, the 170v or higher voltage goes from the SDB straight to the displays, it does not go through the MPU. Your data pins however will come from the MPU. You should be able to unplug displays and still have the others work, just DONT do that with the power on. So you can test with just a single display to make sure they work. Once you get power to the displays, you should get some neck glow to show that they are working even if they dont display any numbers.

Since you found one pin on the wrong place, I highly recommend checking every connector to verify the pin outs are correct.

It sounds like you may have dodged a bullet with that wire in the wrong spot.

If the flashing lights you are referring to are in the head, those lights are controlled by the aux light board.

If none of the rollovers are working, they are stuck off, right? The 3 top rollover switches are in the same column, along with the targets. Are the targets working or not? A4J2-3 is the connector pin number for the entire column, so if none of the targets or rollovers are working, that would be the connector to check. White wire with blue stripe. Also, check each switch to see if this wire is making a bad connection somewhere. The switches are daisy chained so one bad wire can affect them all.

Its a code. A4J2-3. Means board A4, connector J2, pin number 3

A2 P.S. = Power Supply and Regulator board.
A3 S.D. = Solenoid Driver/Voltage Regulator board.
A4 MPU = MPU board.
A5 L.D. = Lamp Driver board.
A1 Dsp = Display Driver boards connected to the score displays.
A8 Snd = Sound board.

You should read the tech guides, that's where you find all this great info . http://techniek.flipperwinkel.nl/ballyss/rep/index1.htm

At MPU board connector J1, the 4th wire (green in color) from the top activates the third digit from the left at all displays. Sounds like you need to re-terminate that green wire at that MPU J1 connector.

It's in the Space Invaders schematics you can download from ipdb.org - the page in question is attached below: The wire colors are represented by two digit codes. First digit (left) indicates the primary color, second digit (right) indicates the stripe/band color. So eg, a connector wire listed with "74" is an orange-green wire. I pasted the wire color code table at the bottom of that diagram.
SpaceInvadersAuxLampBoard.png

Third digit from the right is the third wire (yellow) from the top.
Display digits are counted from right to left on these systems. So first digit from the right comes from the wire at MPU J1 pin 1. Sixth digit is the wire at MPU J1 pin 6, etc.

Download the manual and schematics from the documentation section here if you don't have them:
https://www.ipdb.org/machine.cgi?id=2252

Make sure the contact is 100%.

This may or may not be helpful, but I just got the chaser lights working on my SI this weekend after a lot of repinning and such. It was actually some DeoxIT on the block connectors that finally sorted out the few missing chase lights I had.

Definitely check the lamp power coming off the rectifier board going to the backbox/insert board.

Deoxit can help, but in the long term is just a bandaid. It won't fix connectors that have lost their spring tension or plating either. You really should get the appropriate connectors and repin them all.

http://www.pinrepair.com/connect/

Take one problem at a time and slow down. It's like getting an old car...and with the miles of wiring, switches and things - you may have many problems to iron out and make it reliable.

When you get your general illumination fixed the sound should return.
***EDIT: Disregard the above. Not accurate***

The GI is AC voltage. Sound is run off DC voltage. Most "dead" Bally games of this series will have the GI working and nothing else, so why do you say this?

Oops. My mistake. I was thinking it used the GI to make a -12V.
I'll fix that above...

Made me a little crazy for a moment

This is absolutely correct. In my case I put some deoxit in while trying to figure out my own weird gremlins and found that it was that particular connector that was the reason for all my ghosts. I ordered all new connectors and pins from Great Plains and repinned everything over the weekend and now my SI is finished.

As for the OP it really sounds like a combination of switch and wiring issues. Having just finished chasing the ghosts out of my own machine I don’t envy him. However, I am willing to make myself available for video chat or a phone call if I can be of any assistance. I know I could have used the help lol. PM me if I can be of any assistance.

It's one of those tedious jobs, but its cheap and makes troubleshooting any future repairs *easier* if you are not dealing with flaky intermittent connectors.

When I had my Space Invaders, I replaced every connector on the rectifier and SDB. I also replaced the connector for power to the MPU. I also replaced the connectors to the score displays because they were flaky too.

Repinning and replacing all connectors is a.good idea, but not always necessary...but I do recommend at minimum replacing all critical power connectors.

I did this on a Silverball Mania, same generation of Bally, many issues went away after the job. It's just something that has to be done on these games.

Yes, I ended up replacing every single connector and pin, it took so many hours lol. But it did fix quite a few very odd problems I was having.