Hi again, @OldStrummer,
Thanks for your appreciation for my help. I'm sorry that you are having such a hard time with that Dell server 
You mention that you suspect that you may not even have a GRUB menu, because your system is running in UEFI mode. I don't think that is the case: my laptop is running in UEFI mode and I have a GRUB menu. I think what it's more likely is that the GRUB menu in your installation is hidden.
First, I like your idea of making a bootable USB drive with Ubuntu MATE 20.04. If I were you, I would start by doing that, so that you have a way to boot the machine if other problems arise.
Then - assuming that you already have a good backup - my suggestion would be to edit the GRUB configuration to be able to see it . However, as you're using Ubuntu MATE 20.04, I've searched and found the following topic here in the "Ubuntu MATE Community", started by @tomec on 14th February 2020, that reveals that you may be hitting / hit a bug, when you try to do that:
In that discussion (that had/has several replies),@franksmcb (Bill) wrote the following reply that mentions a Bug:
That bug - Bug #1863434 “20.04 grub menu not visible” : Bugs : grub2 package : Ubuntu - was reported by @franksmcb (Bill (franksmcb) in Launchpad) himself on 15th February 2020
Some comments in that bug point to a workaround described in "Get grub menu back after installing Ubuntu 20.04 alongside Windows | by Jerry | Medium" which basically amounts to edit the "/etc/default/grub" file (I suggest that you make a backup of that file before editing it) and, in that file, change the "GRUB_TIMEOUT_STYLE" line so its value is menu instead of hidden:
GRUB_TIMEOUT_STYLE=menu
... and uncomment the following line:
GRUB_TERMINAL=console
Also check/confirm that GRUB_TIMEOUT in that file is set to a positive number:
GRUB_TIMEOUT=10
Finally, run sudo update-grub to update the GRUB configuration and then reboot.
I hope that helps 