Yeah - good luck with all that. I don't think, short of small-claims court - or very honest respectable folk who will acknowledge wrong-doing and step up the plate (which I suspect if they were that, this wouldn't be a topic at the moment)... you may be exercising futility. Futility seems to be pretty strong already, as it is. So, let us all know what the outcome is.

As for the vacuum advance, it is a diaphragm in the distributor which if you hook a hose to and suck on, you should see the points plate move. Should move with very little suction.

If there was too much crud inside the distributor, I doubt seriously it would make it into the vacuum advance, but would probably act more as "grit" keeping the plate itself from rotating. Hard to say. Just "years" of grime could do the same thing.

The only way to check whether the advance circuit is working is to run it and watch a timing light as you throttle it up. You can also check the advance vacuum with a tee inserted and vacuum gauge attached to make sure the carb vacuum source is actually working too.

Sounds like a tech day in the making....