Or could the motherboard be completely fried?
Check for dull solder joints on all SMD caps (they're in little silver cans with black paint on one side to denote polarity). Dull joints would be a sign of leakage.
It's possible the board needs a re-cap anyway even if it looks okay visually, but I'm sure that installing the roms incorrectly wouldn't have done the roms or your motherboard any good at all.
As for the voltages, well you've tried a different PSU and they're both giving you the same result so I wouldn't worry about the PSU/voltages any more.
Can I ask what led up to this fault in the first place? Before the Amiga was known to be faulty, was it working okay for you? Did it happen over night or something?
