Despite it only being only a few years old, motherboards compatible with Rimuru's processor are largely gone and needless to say, my AsRock isn't re-obtainable as far as local resources go. That's been the sad trend IMHO, that yes, desktops are still pretty modular, no the parts won't be worth a fart tomorrow. But alas, that's a different issue.
Deciding that the motherboard is the root of the problem based on my multimeter readings and the screwed up power behavior, I debated two courses of action: decommission Rimuru in favor of a laptop, as it was already expected to be my last desktop build; or attempt to fix things with replacement parts. The upside of the later is that it is the minimal cost option, the former that it's the less likely to piss me off.
Rimuru is now rocking an Asus motherboard a generation forward. A small fortune and the better part of my day later, everything seems to be operational. Fortunately, re-activating Windows licenses purchased from Microsoft's own store are still not too terrible to deal with motherboard replacement.
In the process, I've also decided to ditch the humongous air cooler and get a liquid cooler, cue kraken, stage left. When I originally designed Rimuru, I had considered liquid cooling and decided to stick with what I know. Well, I decided if I was going to be replacing a motherboard, there was going to be something a lot smaller hovering over the processor getting int he way of my hands, or I was going to drive a spike through the board. So, liquid cooler it be.
Now if there was just a solution for the raging headache ^_^.