BuzzPCSOS
in that exhaustive link I posted, somewhere near the end ("CSM uses" section) he concludes "The CSM is a useful stop-gap tool, but it should not be over-used. Unfortunately, many in the Linux community have been doing just that—advice to activate the CSM as a routine part of Linux installation is common...Do leave the CSM enabled if you're installing Linux to a computer that already uses the CSM to boot all its other OSes."
Maybe the age of the Toshiba qualified me for the "stopgap" tool installation. Leaving UEFI enabled, bizzarely, caused the installations not to boot whether Secure Boot was enabled or disabled.
That's why I think half of all Install Failures in laptop are the wrong BIOS moves....and the other half are the minute variables (stuck keys, faulty media, ill-prepared target drives etc) that are so easily overlooked that you discussed in detail.