What voltage does it get? Is it ~6VAC for the lights, or something else?
Sing Along power rails (resized).jpg
Before the ball is served, or after the game has ended and you still have power on the playfield, what voltages do you measure between the black wire on the left and the 4 points shown on the right?

/Mark

Where exactly are you measuring AC voltage? Maybe show us on the schematic. None of those values are anything I'd expect. The 29 volts you measured between the 25 volt rails is more like it. If you remove the two fuses in the 6 volt/GI circuit do you still get those low voltage readings?

Is the O relay active at all times? If there are 29 volts between its solder lugs the coil is getting power and should be firing:
Sing Along O relay (resized).jpg
Are you sure you're measuring the voltage between the black wire and the yellow wire? If that's the case either the Outhole switch or the holder switch on the O relay is stuck closed.

As for the low voltage on the other relays, notice the Zero Position Ball Cont Unit switch shown above. Are all of the relays that show a low voltage above this switch on the schematic? This switch is intended to cut power to most of the game when there are no balls left. It could be that your switch is mangled so that it shorts somehow when it should be open. Or maybe the fish paper insulation on the switch, or the plastic sleeve on the pin on the stepper gear is worn and there may be a high resistance path through even in the zero position. If that's the case the switch and/or the pin might get warm if left on long enough. You could also shut off the power and measure the resistance across that switch in the zero and other positions.

Shorts are often hidden from view so only a meter can show conclusively whether or not they're there.

If you're measuring the AC voltage across a coil you just need to carefully touch or clip your meter probes to the two coil solder lugs. In the case of the O relay one lug is soldered to the common black power rail that probably goes back to the transformer. The other lug is soldered to a yellow wire that goes to a holder switch on the O relay and to another switch on the score motor. You could in theory measure the same voltage between the transformer and the score motor switch for example, but including all that extra circuitry is unnecessary and may introduce more uncertainty into the problem.

Another way to test if a coil is getting power is to use a light tester as described at: https://pinside.com/pinball/forum/topic/em-pinball-circuits-basics-to-not-so-basic/page/2#post-3184715

I think I'm getting into the weeks a bit. There seems to be a lot of extra details that don't necessarily go together like the low voltage measurements. So for the moment lets dig into the O relay. If I understand correctly it fires even after the game ends. That means there must be a path somehow between the red-white wire in the lower right corner, and the red wire in the upper right corner:
Sing Along O relay 2 (resized).jpg
When the game is in game over mode and the outhole switch still works, you should be able to slip a piece of folded paper between the contacts of any of the switches along the right side of the schematic (DS, XB, TB or SB) and be able to stop the O relay from firing. If that's not the case, which of those four do disable the O relay and which don't?
It sounds like you have a short but it's hard to tell if it's through a switch or through something else like the frame of the trip relay bank for example.

Should have started, "I think I'm getting into the weeds a bit."...

What playfield features were live? It would help if you put together a complete list. Then we can see what they have in common.

What did you do to the score motor? Can you undo it or retrace your steps?

I don't follow most of this. Since most things have two terminals you need to clarify which terminals you're referring to, preferably with wire color. "jumpering between the score motor and C" is too vague.

If you manually close either the C or U relays, they should lock on and start the score motor turning. They should relax when the score motor 2B switch opens right at the end of the score motor's 120 degree turn. Do they not do that?