How did you test that Q37 is working? because it's not a transistor, it's a SCR (silicon controlled rectifier) and they function differently.
Can't say I've ever seen a single output on one of the 4514 CMOS decoder chips go bad.

That doesn't tell you the ability of the SCR to latch on though.
First thing to try is grounding the "Anode" leg of the SCR. The respective lamp should illuminate which tells you wiring from the SCR to the lamp socket and the lamp itself are good - note this doesn't test the SCR, only that the circuit from it to the lamp are good.

Next to manually activate the SCR in circuit, grab a jumper wire and connect one end to test point TP3 on the lamp driver board. Connect the other wire end onto the "Gate" leg of the SCR. If the respective lamp illuminates, the SCR is good. If the lamp still fails to illuminate in lamp test mode, then you can go probing upstream on the 4514 chip.

2N5060a.jpg
SCR_MCR106.jpg

How are you getting to solenoid diagnostic mode if you are only getting 6 LED flashes on power-up? The MPU board will stall if it doesn't successfully flash 7 times.

All displays showing '2' on every digit is likely to be a bad connection of the J1 connector pin 26. The 6810 is not the problem.

Sounds like you have an Alltek MPU board.

Failure to get the 7th LED flash means that the MPU board is not getting it's 43 volt input. It comes in at pin 15 of MPU board connector J4.

Get your multimeter out and measure it:
Factory Stern MPU board you should measure 21 volts at test point TP1
Alltek MPU board revision L you should measure 43 volts at test point TP2
Alltek MPU board revision J and K you should measure 43 volts at test point TP7
any other revision Alltek, you need to look in their manual for it.

Sorry, if you get a '2' where there should be a '0', a '3' where there should be a '1', a '6' where there should be a '4' etc, then it's the same issue at pin 26 of connector J1.

Anyway it sounds like you may have a few MPU board connector issues that need to be dealt with.

Voltages look good. So if these are what you read even when the game fails to give the 7th LED flash then you could be looking at an issue with the zero crossing detector chip (U14) on the MPU board or a bad connection at the U10 socket on pin 18 (this is where the zero crossing detector signal ends up). A logic probe will help you in this case.

You're looking for the logic probe to indicate activity on the signals, i.e. both high and low logic probe indicators should be on and if the pulse indicator flashes, that's extra confirmation.

If you get either the low or high indicator on *only* or no indicators at all, then the point you're probing has no activity.

The destination of the zero crossing signal reaches U10 pin 18 and should show all three indicators doing something. Probe that point first and follow the signal backwards via the schematic if you know how to read it.

Disconnect all but one of the displays. If it still happens, swap that last display for another. If it still happens, you have an open circuit on the display BCD data 1 signal from the MPU board at pin 26 of J1 to all the displays at pin 18. This is a Blue-White wire that gets daisy chained from one display to another on pin 18. More likely you have a bad crimp terminal connection at the source MPU board pin 26 of connector J1.

I don't own a Seawitch but if I did, I'd probably move the left post a touch as mentioned here:

https://pinside.com/pinball/forum/topic/any-seawitch-fans-on-pinside/page/16#post-5451065
https://pinside.com/pinball/forum/topic/any-seawitch-fans-on-pinside/page/16#post-5455268

And also experiment with changing that upper left flipper for a 2 inch bat.

These games don't have a lamp matrix so ghosting isn't an artifact of using standard LEDs with the way lamps are illuminated on this architecture, only flickering is. You have some other problem. Are you getting wrong lamps illuminating?

This sounds like the wire from the anti-flicker boards is not making contact with the 6V feature lamp supply voltage. What voltage do you measure at the hookup wire on the anti-flicker boards? Where do you have this wire hooked up to in the game?

Turret / Terminal strips?

Not 100% the same but workable:
ebay.com link: itm

Terminal-Strip-3.jpg

https://www.digikey.com/en/products/detail/tubedepot/TB-U3/10490945
Terminal-Strip_MFG_TB-U3.jpg

What is switch test mode telling you about these switches?
Does it happen when all drop targets are up? If it only happens when certain drop targets are down, exactly which ones?
If you start game and then remove the J3 cabinet switch harness from the MPU board, does the spinner issue still occur?

The spinner causing other switches on the same row to be sensed is usually a short on the strobe column wire to something like a playfield lamp.
Someone recently who did a playfield swap had a similar spinner issue causing ghosting in their switch matrix. They accidentally had a lamp socket touching a T-nut which their spinner post was screwed to and through the spinner frame electrically connected to the spinner switch.

Try this for me, rotate the spinner 180 degrees so the spinner switch is closed and somehow lock it in that position without you touching it.
Turn the game on and see if the MPU board fails to boot past the 3rd LED flash.

Also carefully inspect the white-red wire on the spinner switch and top rollover button (both on the strobe #0 signal).

That's because these games don't have a "lamp matrix". It is not the Williams columns/row lamp system. Each lamp has an individual control device (SCR). The lamp driver board schematic has all the wiring info needed per @cottonm4's post.

Someone installed Williams style flipper bats in the two upper positions when I got it my game. Normally I would not stand for wrong bats but to me they make sense on Seawitch with the geometry. I haven't played it with Stern upper bats so am afraid I can't compare for you but it does slightly open the shot from the lower right flipper.
It's something you can try that's easily reversible.
I've even thought of installing a 2" upper left flipper bat.

Following on from what you did above, at some point I'll be trying one of these narrow blue posts on the left riptide entry.

Posts_Slim_03-9357-xx_blue.jpg

Does it play a sound effect when you drop a coin?
Is the coin chute configured to require 2 coins per credit?
Have you already reached maximum credits?
Do coin chutes 1 and 2 work properly?
Post a picture showing the 32 MPU board DIP switch settings.

The only reference I've seen was posted by Mad_Dog_Coin_Op
https://pinside.com/pinball/forum/topic/any-seawitch-fans-on-pinside/page/23#post-6367673

My game has most of the plastic posts in the wrong positions and I've been meaning to see if Mad_Dog_Coin_Op list matches the number and sizes I have but haven't had time.