i've allways waxed right after the final polish, never had issues, and the paint on both my cars is rock hard, but i never intentionally tried a scratch test on my cars, only reciently when the whole scratch debate started did i test the charger, which was painted 5 months ago.
as for the wetsanding, i clean the residue off while it's still wet, just after wetsanding, i use a shammy and a bucket of clean water, it's alot eaiser to clean while wet, then just before painting, i clean with mineral spirits, and then use a standard autobody "tach cloth" which is a sticky cheese cloth rag you can get at any auto parts store for $1, before i paint each pannel/area i use the tack cloth, and have virtually no dust/dirt issues.
as for painting over plastic bumpers, i've never had any problems with any parts i've painted, but like any paint if it's flexed enough it will crack. i've painted my 85 CRX bumpers, and my old 86 mustang gt, no issues with the paint sticking or cracking. the front fenders on the charger are fiberglass, i've popped them in and out while working on the engine by accident and nothing happened to the paint, in the sun when those fiberglass fenders heat up they get about 1/4" longer and expand from the heat, no issues.
Charger... Did you do anything special when you prepped and painted the fiberglass fenders (prime)? I assume you received them and they had just a standard gelcoat, right?? I read that it (gelcoat) has a waxy type consistency/coating... Did you treat them like you would have a metal set of fenders? The buggy I'm soon to be painting is all fiberglass.
Sorry for all the ??'s, just trying to get some similar results. Thanks for all your insight!!
