It's been a while since I even worked on this so bear with me while I get my thoughts together.
Regarding the desoldering: the board I was working on had a heavy amount of oxidation (not corrosion) on all the pins and solder to the board. It took quite a lot of time, flux, and combination of regular iron and sucker desoldering iron to even free all the the pins up. Resoldering took no time at all due to not having to deal with that. There are 3 or 4 boards which needed repair probably 25 years ago that were part of some guy's repair shop and just never thrown out. Trust me, no though here of questioning my soldering skills. If I can learn a newer/better way of doing it, I am still a beginner with all this so I always appreciate help and tips.
The reason I was originally suspicious about the chips is that when I started up using that board originally, I got a chime, gray screen, and then the RAM test error with a sad Mac. After replacing the chip the first time, with no modification, I got no chime and no screen activity whatsoever...so it got me thinking that I must have messed something up with the RAM or the RAM itself was bad because going from RAM test showing a bad chip to NOTHING was concerning to me.
Another distinct possibility is that I somehow reversed the chip entirely. These chips have no notch or dot denoting the top or pin 1, so I went with a guess based on the orientation of the printed writing on it. If you know for certain what side has pin 1, please correct me.
I have one 512ke in my possession that works and has never been modified to my knowledge, so I can certainly use it as a test board to verify the +5v /0v to verify the pins in a working machine.
I still have probably 15 or so chips to use. I bought them to replace bad chips on all three boards. In this instance, I will use them to make sure that we all can figure out this mystery for certain (though I hope that does not take 15 good chips destroyed!).
My last thought was this: to make this a lot easier, I am just going to solder in mini sockets into the RAM chip spot. This way, we can test whatever we need and see what to do next without me taking forever to do it.
Thanks for the response @Byrd, just given that a read, looks like a neat idea.
I've looked into the revisions, it would appear that both of these are Rev 2s, as the CMD chip is labled '646U2-402' as opposed to 'PCI646U2'.
I also tried putting the internal Zip over from the dead machine to the working one. Weirdly, when I tried that, it turned itself on when i plugged the machine in and refused to chime. Taking it out again sorted it out