As a follow-up to my previous post, I did a half an hour of investigation, and here is what's happening: occasionally and for reasons no one can discern, the old DOS utility "chkdsk" will pseudo-randomly re-assign "drive letters" when it runs, on systems where dual-booting is leveraged. I booted to a "safe mode" DOS prompt, and confirmed with "diskpart"-- that is indeed the issue.
I tried re-assigning them, only for them to flip back again-- in other words, Winbloze still won't boot. I found some commands which can be run via a USB "safe mode"-formatted stick, but at this point, how can I trust Winbloze 10-- is this going to happen every time there is an update from here on out? It's not even worth the effort, since I've barely played games on my PC over the past few years anyway.
In general, I've been playing PC games since the mid-80s, but this might be the end of that road: Microsoft is incapable of putting out a usable, non-bricking OS, and publishers don't make many games for GNU/Linux.
And unlike those heady days of the 1980s and 1990s, PCs simply share their entire libraries with the dedicated systems, so why not just play on those? I'm tempted to hand my RTX 2080 to the wife, put the old GTX 1070 back into this PC, sell the whole works on eBay, and buy a cheap non-gaming laptop: as long as it can run Manjaro and RetroArch, that's all I need.