Quoted from acebathound:
With the NVRAM out of the board, take some close-up pictures of the front and back of the 5101 socket. There might be something someone catches in those pictures that could help you out. Otherwise, if I was you I'd use a meter on continuity test and beep things out at the 5101 socket -- without any RAM in there. Use the schematic to verify connections and check adjacent pins for address/data lines that SHOULD NOT have a connection.
Thanks @acebathound , I appreciate that write-up. Sadly, pressing the daughter board doesn't help at all, so I guess there's a solid that's that's not it.
I've pulled the board several times since I installed the socket, and tested everything. It seems good, but maybe it's worth a third go around. I'm pretty sure I've checked every possible mistake/issue, and "beeped" out everything with my meter, but it won't hurt to do it again. Perhaps some pics wouldn't hurt, since it will give everyone a chance to see if I'm visually missing anything. I even searched the board for solder splashes and such, no luck.