I managed to get it installed. Permuting BIOS settings, removing the NVMe Windows drive, booting safe mode graphics, nothing worked. I tried thee different brands of USB3 sticks and verified the sha256 sig of my download. Wrote the image with two different versions of Balana Etcher and 16.04 "Disks" utility, no joy.
"Try without installing" always ended hung with a black screen or partially drawn desktop.
Any error messages flashed by too fast to write down, but something very early about initramfs not decompressing, and some wifi firmware feature having GWS but not GFS or some such.
I don't know what possessed me to try, or if it was menu fumble fingers, but booting directly to the installer worked!
I now apparently have a working 20.04 Mate system running. So far its different, but not too strange.
This laptop has some "funky" touch pad that 16.04 installer couldn't use so I needed a USB mouse, but 20.04 installer worked fine with it, and the "multi-touch" seems to work really well for scrolling Firefox etc. after installation. Correctly setup the Nvidia propertiary drivers and seems to have activated the WiFI (although I don't use it at home, so I've not connected to it)
I put the BIOS settings all back to where they were originally, and nothing changed. All this makes the abject failure of "Try Ubuntu without installing" even more infuriating and mysterious.
How many bad impressions will this issue make among potential new users who want to try the USB boot before installing to be confident it'll work?
I sure hope that what ever the issue is that its confined to a very small subset of laptops. My ASUS fx705gm is I'd call upper mid range, but 16.04 installer worked perfectly on it except for the multi-touch touchpad not being found, although it worked after 16.04 installation was done.