Wow... some stuff has been done to that board! The extra RAM chips are definitely post-purchase. If they weren't, all 8 chips would be the same part. There were tutorials kicking around BBSes and magazines back in the day that explained how to add the extra RAM chips as a slightly cheaper alternative to a trapdoor card (for those with the technical capability). I'm guessing that's what was done here. I'm no expert at reading part numbers, but all eight chips contain the code 514256, so I take that to mean they are functionally the same, but from different manufacturers, of course. The aftermarket chips are 80ns and the originals are 100ns.
I'm guessing the original owner did that mod and then laster wanted some "Fast" RAM, hence the Power Computing card. Did you remove the Gary adapter when you took the trapdoor card out? If not, that might explain the false reporting of an extra 1MB of RAM - some address line being triggered by the adapter that shouldn't be. I wonder if there's also some jumpery stuff going on, since either the extra 512K onboard or the bank of 512K of Chip on the trapdoor card would need to be disabled. Enabling them both would cause a conflict - maybe that's the yellow screen you got?
Anything on the underside of the board?