While not answering your question directly, the rubber size details in red below might help.

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No blue-brown wire listed - sure it isn't blue-orange which is the switch return wire on the Tilt?

It's not used on Quick Silver.
Earlier Sterns had the red-green wire hooked up to the Slam switch.

It goes to the Tilt, Slam, Start and three coin chute switches. Basically all the front end cabinet/door switches. It's the switch strobe signal that will connect to the diode (non-banded side) on these switches.

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Your connections on the volume control are good enough for it to function, i.e, one wire on the middle lug, the other wire on an outer lug.
Your problem is either the volume potentiometer itself, a connection issue with those purple wires back at the sound board or a fault on the sound board itself.
For clarity, hopefully someone can confirm how those purple wires connect to the volume control but technically the way you have it doesn't matter.

Speaking of which, in the speaker picture by hisokajp above the red spot on the speaker near the right side lug indicates that's the positive "+" lug. You have the speaker wires crossed over though in a single speaker system it makes no difference. Just telling you incase you're OCD about these things. i.e. from factory:
Green-Yellow wire to the positive speaker lug
Green-White wire to the negative speaker lug

Correct, the volume pot should be 25k ohms.

Have you tested it with your multi-meter?
Meter set to resistance mode, set it to 20k or 200k ohms if your meter isn't auto-ranging.
Machine OFF
One meter lead on the middle pot lug. Other meter lead on the outer pot lug where the other purple wire is soldered.
The resistance reading should vary between 0 ohms and 25k ohms as you do a slow sweep across the full pot rotation range.

BTW, where do the purple wires connect on the sound board? They should be on pins 1 and 3 of the J3 connector.

You've done this before right? Does the copper oxidise over time? I'd put a thin layer of solder on the copper surface to tin it where you're mounting the lamp sockets for better long term connectivity.

Agree. I wouldn't trust the adhesive for conductivity, solder the joints.

With the game OFF and the drop targets raised, are the diodes measuring ok on all the drop target switches or do you see any measuring a short circuit?
Post some pictures of the drop target switch stacks from side on.