1) Any suggestions for cleaning the plate on a terminal board that spent is full of dust dirt and God knows what else?
Datavac, compressed air, brush it out, scrub it with a toothbrush, rubbing alcohol, etc. There are a ton of ways to do it. Find what works best for you without losing parts.
2) What is the prefered method for cleaning really dirty non yellowed caps?
Everyone has their preferred method. Some toss them in a bag into the dishwasher. Some use denture tabs. Some use soap and a brush. All work well.
3) Is there any way to tell ABS from PBT without destroying the cap?
Get salt saturated water and toss them in. They float they're ABS, they sink they're PBT.