I think I made another bit of progress with my RAM troubles: trying XMP again, I decided to set the the timings to SPD values manually, with an addition, setting CAS Write Latency to 8 instead of the default 7. I got a boot at the rated DDR3-1600 7-8-7-24. Running a round of Prime95 Blend to see if it is actually stable: if it doesn't fail to quickly I'll leave it overnight, and if it is indeed stable, we'll have a clear culprit, tCWL.