I had the same problem with a little corrosion on the CPU and getting the black screen. Then I had the bright idea of spreading the CPU legs out a little so thay fit may more snug into the socket. I aso cleaned the legs a lot. Since then I have had success with a GARY and DENISE doing the exact same thing. I'm not saying its going to solve anything but its worth a try if you haven't already.
Good luck.
PS The A2620 is probably a red herring. Just leave it out till you get the computer working with the 68000. I assume you checked all the traces around the battery area.
