Are you sure the 100 pt relay is energized? The score reel should be getting impulsed from a score reset relay during the startup sequence (score relays should be isolated from the drum units during reset by gameover switch, unless you have a rogue short) - maybe the score reset relay switch for that drum is closed or shorted and, therefore, not cycling.

Have you checked all switches on the playfield which score 100 points ? One could be closed. The spinner between the pop bumpers could be likely.

Are the switches on your index cam opening and closing properly? No touching tabs or wires on the back side?

So, the score motor is turning over endlessly, correct? Is the Game Over Relay tripped (it should be during reset)? And if it's tripped, it has a switch that should be open that cuts off power to all scoring relays including the 100 pt relay. If that switch is open (which you can verify by trying to press a 1 pt or 10 pt target on the playfield - you should see no response), then I would say you have a short to the 100 pt score relay.

Usually, if you have a stuck scoring switch, it won't reveal itself until after the reset is complete (because the scoring circuits should be dead during reset).

I don't have this game, but assuming the Game Over Re. is on the Relay Bank, then there would be power to the scoring circuits when the score reels are resetting on this game. So, sequence looks good, and I know you said you double-checked all the switches were in the correct position, but it really looks like one of the switches is the culprit. A switch can be open but a path for current can still exist - bent solder tab, stray wire strand, solder splash bridge, etc. There are a lot of circuits to fire the 100-pt relay...

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Remove the single wire going to the 100 point relay coil. If the game now resets properly, you've verified the reset circuits are good. Next, troubleshoot why the wire you temporarily removed has power if all playfield switches are verified as open.
Use standard electrical troubleshooting techniques....

The odd thing about this is that when the machine is resetting, the circuit connected to the 100-pt Relay should be dead via a make-break switch on the Reset Relay (see below). As I said before, these stuck-on score reels usually don't reveal themselves, until after the reset is complete, but you have clearly stated that the coil comes on immediately after pushing the replay button (you didn't say but the score motor should be running continuously). If one of the circuits shown in post #9 above is inadvertently closed, the Reset Relay switch would also have to be closed to get power to 100-pt relay. You never did say if pressing a 1 or 10 pt contact on the playfield during reset has any response - that would tell you if that Reset switch is properly isolating power. If it is isolating power properly, then that could mean you have an unconventional shorting path between power and neutral for the 100-pt Relay - i.e., a path not depicted on the schematic.

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?

So, your machine is and has been fully resetting the whole time (score motor not running) and the Reset Re. switch is isolating the circuit properly. This is good - I just wish you would have disclosed this earlier, especially when I had asked you twice!

So, you just need to find which one of those circuits in post #9 is causing your 100-pt relay to lock-in. Stay focused on finding that for now. Your eyes are the best tool for finding the problem - not a meter. One thing you miight want to check is the jones plug(s) connecting the cabinet to the backbox - could be a strand of wire between pins. There could be a strand of wire or a piece of solder laying across 2 blade switches on a relay. Could be two solder tabs touching together somewhere. It's an Easter egg hunt, but you will eventually find it, and you will end up saying that was easy and I should have seen it earlier.

Curious and have to ask - does the game try to reset then? Because the Replay Re. is what energizes when you press the Replay button to start a new game. Are you sure it is happening at the end of a game? Does it happen if you have no credits on the game?

Score motor cam 3 switch A pulses the tilt relay. Clean switch and check gap.

Everything in that sequence looks normal, except the Tilt Re. not tripping at the very end. You don't mention the Replay Re. energizing anymore like you did in Post #16, so I assume you misspoke and that isn't happening.

When the Gameover Re. trips it starts the score motor running (see blue highlighted circuit below). Also, when the Gameover Re. trips, it has a make-break switch on it that completes a circuit to trip the Tilt Re. as the score motor switch 3A pulses (see the purple highlighted circuit below). Now back to the blue circuit - there is a Tilt Re. switch that opens when Tilt Re. trips and that stops the score motor.

So, you need to check/adjust the 2 switches in the purple circuit that are responsible for tripping the Tilt Re. One or both (not likely) of them is not closing to trip it.

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You just checked one of them (motor 3A). If adjusting that one changed the Tilt Re. trip from "never" to "sometimes", I suspect you have found your bogey and you just need to clean/adjust it better.
Just to be sure, that switch is supposed to be open all the time, except when the cam notch is allowing the switch stack to fall, then it must be closed.

Get your soldering iron out and open circuits (ref. post #9) at strategic points to isolate which branch is causing the 100-pt relay to energize.

I will assume you got the Tilt Re. problem resolved.

What makes you think it's that pop bumper? You should have isolated it from the 100-pt relay when you desoldered that leg in your test. It is lit and worth 100 pts at only 2 out of 10 positions on the Advance Unit, otherwise it is worth 10 pts. Circuit is shown below( red=100pt, green=10 pt, blue=common). I don't think you did the isolation test correctly for all the branches of the circuit.

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I've had the "end of stroke" switch on a 10's score drum unit bent in a way causing the 100 point relay to energize. Since the switch is part of the 100 point relay coil circuit, I assume you cut/desoldered it already. If not, try disconnecting it.
Ive also had the screw insulating tubes on switch stacks (on score reels) fail causing strange problems such as this.

Has the problem been isolated to the playfield by unplugging the playfield jones plugs?

Desperate times call for desperate measures. Isolate circuit at each number sequentially on this diagram (all wires will be brown with grey tracer). You better have isolation at Point #1 or there is a short somewhere directly on the 100-pt relay. If the relay doesn't energize, then you MUST have a short somewhere in the red circuit. Solder #1 back on and go thru the rest of the points 2 thru 11, unsoldering the brown wire with grey tracer at each point until the 100 pt relay does not energize. If you unsolder them all and the 100pt relay is still energized then you have a short somewhere directly in the brown wire with grey tracer.

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Which switch was it and how was it failing? Maybe someone else can benefit from your experience if you give the details.