Update: in the endgame now (at least as far as memtest86 is concerned). Both RAMs work fine in their default factory config w/o XMP and got another pass. Now for the final run of both in XMP profile.
Hope the cleaning / configuring the right voltage fixed any issues I was having. Will know in couple hours.
Hopefully this is a good sign whatever I touched worked to fix the issue. Will do some gaming and misc. tests to see if everything holds.