15th attempt reinstall, enabled Ftpm-everything, cleared it cleaner than a windexed window.
Installed ".net" 3.5 and 4.8 (because some guy said to)
Did I mention it installs 100%, but then UNinstalls??
Yeah.
....(TLDR)
MARK all other drives OFFLINE for immediate success.
35 years of windows installation-programs and NO one else thought to program the installation file for multiple drives???
O ... My ... Gosh.
Can I wildly speculate?? Because I'm mostly talking to myself anyway...
My data-drive (an nvme) is technically drive 0 and the System drive is technically drive 1.
When Preboot reaches out to "drive 0" it melts, has a brain-infarction, whatever.
If I tore my pc apart and removed one of the drives (the data-nvme) it might not faint.
But tearing apart a pc is already risky, and with no confirmation, I question whether it's worth the trouble.
Success!! Offlining The second Drive helped!
But did I need to do that? (yes)
Couldn't windows *know* where the C drive was? (no, apparently)
Did any one else have this solution?? (It should be a sticky)