While the online manual is missing the playfield schematic, as slochar said the wire color info for the switches is actually there. See the first diagram.
The second diagram is the playfield switch schematic that frunch posted here at some point or another.
Last diagram is the lamp driver board schematic if you need it posted by @gabegabegabe.

[Edit] Actually the playfield switch schematic and lamp driver board schematic came from this thread - there's also the playfield lamp info there too:
https://pinside.com/pinball/forum/topic/wanted-schematics-for-cosmic-princess-nine-ball

NineBall_Switches.jpg
NineBall_SwitchMatrix.jpg
NineBall_Lamps.jpg

Sorry I've never seen or played the game so have no data here.
Why don't you post pictures of those switches in the left ball lock area clearly showing how they're wired in case we notice something obvious?

Ahh crap, thanks for the reminder. Somewhere I modified another lamp driver board schematic and made one for Nineball - now I gotta find it..

See below:
Nineball_Slingshot_Wiring1.jpg

Yes, diode orientation, capacitor and white-yellow wire locations look correct.

What are you trying to fix?

I had this discussion with someone here a few years ago who was certain that Stern fitted capacitors on the slingshot switches. Stern schematics don't list them though. All my Sterns are in storage so I can't check.

Go to switch test mode.
Raise all the drop targets and remove all balls from the playfield first.

The game should report "0" in the ball in play display indicating no closed switches. If not, let us know what switch number the player displays are reporting.

Using the switch identification tables in the manual (paper pages 17 & 18, PDF pages 18 & 19), activate each switch one at a time but do it in REVERSE order listed in the manual - i.e. start at switch 40 and work your way in reverse sequence to switch 1. It's important that you do it in reverse order because the game only reports the lowest switch number that's closed.

Leave a ball lying on any outhole/saucer switch after you activate it, ditto with the drop targets leave them down after you drop them.

Let us know if and which switches indicate the wrong number so we can try and work out if there's a switch matrix issue.

In game mode, does the ball get stuck in the left lane saucer even when ball capture isn't lit? i.e. when it's not supposed to be locking the ball?

What happens in game over mode if you take a ball from the outhole and roll it into the left lane saucer? Does the game kick the ball straight out? it's supposed to, in order to make sure all balls are back in the outhole trough area and ready for new game. Or does the ball just sit in the left lane saucer?

Ok, I think there's a problem with your #7 drop target release coil.

In your video above when the ball enters the left lane saucer, the game should drop the numbered drop targets in reverse order starting from number 7 but yours is not doing that. Then when you manually knock drop target #7 with your finger and the game detects it, the game then resumes dropping the other targets as it should and then finally kicks the ball out of the saucer.
If any of the drop target release coils don't work, the game will sit there waiting for the respective drop target switch to activate before proceeding.

So go into solenoid test mode and check that the drop target release coils for drop targets #7, #6, #5, #4, #3, #2 and #1 are working.

That happens in emulation too so it's probably normal when the ball is in the saucer.

[EDIT]

Oops it took so long for me to write my reply, I think you already found where the problem is coming from..
[/EDIT]

Coincidence?..
In solenoid test mode it's solenoid number "13" for the #7 drop target release.
Driver transistor Q13 on the solenoid driver board drives that coil.

I've whipped up a rough Solenoid Driver Board schematic for Nineball (borrowed template from the Lightning schematics).

Nineball_SolenoidDriverBoard_repro.pdf

There's no release coil for drop target #8 so that's normal.

Onto drop target #7. Quickly inspect the drop target mechanism where the #7 coil is to make sure there's no wiring issues.

Next, grab a piece of wire with both ends stripped. Hook one end to ground and *very briefly* touch the other end on the metal tab of driver transistor Q13 on the solenoid driver board. Does the #7 drop target fall? If not, try grounding the metal tab of other transistors i.e. Q14 should make the #6 drop target fall, Q9 for drop target #5 - just so you get a feel for how grounding the transistor metal tabs behave.

Sorry I don't have a Nineball - I'm simulating your behavior in emulation and studying the schematics.
Having said that if you ground the metal tab of the Q13 driver transistor and the #7 drops then you know the #7 drop coil itself is working and you have continuity from the solenoid driver board to that coil so wiring should be ok - anyway the schematics say it's a black-yellow wire that comes from connector J5 pin 12 of the solenoid driver board to that coil.

Need to find out why the Q13 driver transistor is not firing.

Grab your jumper wire, hook up one end up to test point TP6 on the solenoid driver board. Very briefly touch the other end of the wire on the banded side of diode CR13 on the solenoid driver board - it should be near the Q13 transistor. This will test transistor Q13 itself. What happens? Does that #7 drop coil activate?

Sorry, please do the ground test of driver transistor Q13 on the solenoid driver board as I mentioned back up in post #64 and let me know what happens. For some reason I thought you already did it.

This is not a switch/MPU board issue.

Now go and do the jumper test I mentioned in post #71

Ok, next grab your jumper wire and connect one end to ground then very briefly and carefully touch the other end on pin 11 of the U3 chip on the solenoid driver board. If you're not sure which is pin 11 of U3 then please post a picture of your solenoid driver board so I can point to it.

This is nothing to do with switch matrix issues.
Once the ball activates *either* the top left lane switch or the left lane saucer switch (they built in redundancy here), the game goes into the merry dance of dropping the numbered ball drop targets. Your game is clearly seeing one/both of those left lane switches activate by the sound effects it's making.

Are you sure your Bally game that you took the Bally solenoid driver board from actually uses the Q13 driver transistor? Not all games use all the transistors since not all games are fully populated with 16 momentary solenoids. While it would be unusual for two boards to have the same fault, we have to discount it.

The problem is wholly that the #7 drop coil is not being activated after the game recognizes that the ball has entered the left lane and we are trying to diagnose why.

Which Bally game did you pull the solenoid driver board (SDB) from?

Your video in post #69 shows me you don't have any connector issues between the MPU board and solenoid driver board with relation to solenoid selects.

I've tested both the Nineball ROM on IPDB and the freeplay version floating out there and they both show when the ball goes in the left saucer that they repeatedly activate that #7 drop coil every half a second waiting for the #7 switch to activate so it can move to the #6 drop coil. So from a software perspective, the ROMs easily available for download do not have such a bug that you're experiencing from what I'm seeing. Olivers homebrew Nineball ROM behaves very differently and your game isn't behaving with his ruleset for the saucer.

So burn the ROMs from IPDB if you really think it's your problem. Otherwise give me feedback on post #78

Sorry I just watched @redketchup's video.
@barakawins1 when you follow exactly his routine of dropping the targets in sequence, after you drop target #7 and the drop target banks reset up, is your drop #7 coil activating after #1 to #6 have dropped in sequence leaving only the #8 drop target up?

Check that you've got zero ohms continuity from the metal tab of Q13 through to connector J5 pin 12 of the solenoid driver board (black-yellow wire) and finally to that #7 drop coil.

Yes, drop target #4 needs to fall just like the other drop targets #7 through #1 before the ball gets kicked out of the saucer.

Both the Bally and Stern solenoid driver boards are functionally the same. Both your boards have faults.

Grounding Q10 does not test the transistor so if the drop #4 fails to fall when you ground this transistor you have an intermittent connection issue which is likely a cracked solder joint on this solenoid driver board either on the Q10 legs or at the J5 pin 15 pin header (very likely as the solder joints on the edge pins of this pin header often crack). The pre-driver circuit is upstream from this Q10 transistor so they are not the issue.

Possibly, but first check that diode CR13 is good with your multi-meter - compare its results to one of the other diodes. The U3 chip activates the Q13 driver transistor through diode CR13. Note the U3 CA3081 chip is is just a transistor array. You can actually test the individual transistors inside it with your multi-meter before pulling the chip out. Refer to the solenoid driver board schematics I posted above that shows the U3 CA3081 pre-driver transistors individually. You can compare multi-meter diode mode readings to other transistors inside this chip.

Capacitors are only used on fast activated switches to help the CPU see them activate. They're typically on pop-bumpers, standup targets and rollovers that the ball travels fast over. They're not needed on trough switches.

If you're doing that with your other hand touching a metal side rail (that's wired to ground) that behavior is not unusual.