SO, a pull (I have a proper puller) and clean the chip and socket with ISOpropo and we are back in business with the blitter, it is a non atari blitter btw, the kind that exxos sells, so, wonder if this machine has had issues with a blitter in the past, or on later boards they were using generic blitters.
I don't think that Mega STE-s were shipped without blitter, since it was introduced 2 years earlier.
Considering EPROM programming - I guess that you should try another EPROM chip. Or maybe just to erase it little longer. Really, there is no such ROM content what is bugger - there are only 0-s and 1-s everywhere, and every combination (there is 2 power 1048576 of them
) can be programmed in EPROM without problem, when all is OK - EPROM and programmer device + PSU for it if needed.
Negative feedback has usually positive effect.