I appreciate your caution, and your posting the schematic, but I don't think it has enough resolution to read all of the labels. If you get a better scan consider sending a copy to IPDB.org too.

But you can see that the Score Motor has 8 switches in addition to the motor run switch that can keep it running. I'd investigate and/or block those switches to see if you can identify which one is closed or shorted.

The Lock relay should be active whenever there is power to the transformer, which switches on the Lock relay should keep flowing once activated. It's possible that some other large solenoid is trying to fire, perhaps longer than it should, that might drop the voltage enough for the Lock relay to let go.

/Mark

I'd investigate that a little more. There is a big Bank Reset coil (in the red box) that is activated by Score Motor cam 4 that probably happens late in the Score Motor cycle. You could try blocking one of the switches that drives it with a bit of paper to see if the Lock relay drops out if the Bank Reset coil does not fire.
Naples bank reset (resized).jpg
Beware that that coil and everything above the transformer in the schematic is running at 120 volts so you should unplug the game before making changes.

Another option would be to block the switch on the Game Over relay that provides power to the bottom half of the schematic. That would keep many other coils from firing.

Beware that unless they've overheated or been abused, coils don't usually go bad.

Score Motor switches on Williams schematics are drawn with a circle around them, so in your case the switch in red below keeps the motor running. It closes as soon as the Score Motor starts turning and opens when the motor reaches the next home or index position. The "Index" label on the switch indicates that it is operated by the Index cam.
Naples score motor (resized).jpg
If you have a Score Motor that looks like the motor below then the switch in question will be in the switch stack operated by the zero, or more commonly called the Index cam which is next to the motor itself. The motor run switch is usually at the top of the switch stack.
Williams Score Motor (resized).jpg
The motor image above is from Williams' Introduction to Coin Operated Amusement Games which is a handy reference available in various places online including:

https://www.funwithpinball.com/resources/parts-catalog-list-page-2#Williams

Are you referring to the letters in the wire colors?
Naples wire colors (resized).jpg
Their use seems inconsistent to me. Note the A-21 and B-21 in the upper left for the same wire and the 35-X and X-83 below the transformer. It's not a notation that persisted in later schematics so I have to wonder how useful it was.