To mount modern derailleurs on skinny vintage 28.6 down tubes, use braze on adapters and the braze on type derailleurs. The adapters cost $20 maybe.

For the stripped out Campy FD, if you cannot tap the hole to a larger bolt, perhaps you could drill the hole through the derailleur arm, then use a longer bolt and nut. A bit ghetto but who's going to look that closely?