Well, damnit, I made one teeny, tiny little mistake in my otherwise perfect surgery. I had yanked my CPU fan connector from the board to tuck it behind something else, and plugged it back into the pump_fan header.
It was working, but it became "fan2" and it was running at slightly lower RPM than I remembered it (like 1490 vs. 1500+). I thought oh well, I just plugged it into a different fan header and the sensor is reading differently or something. So I went to the BIOS to see how it was reporting fan RPM and it was 0 RPM for the CPU fan. That's when I knew it wasn't a CPU fan header I plugged into
But... that was easy to fix without moving the machine much or disturbing anything else, it was just a pain to get it on the pins, reaching in there.
P.S. Actually the fan RPMs are about the same, plugged into the correct CPU fan header. It's probably the PSU that's changed that (and I'm not using Smart Fan Control). I also noticed my CPU voltage in the BIOS is staying lower than 1.4V now.
I think I have a good case for returning that PSU without much embellishment. I'm going to say "It has poor grounding/shielding and EMI issues as well as imprecise voltages. It caused me a lot of subtle problems with my build, and a rebuild in a better computer case, until I replaced it."
(Note that imprecise voltages thing is always true to a degree, with Antec vs. Corsair PSUs. I don't lie

)