Re: timeshift -
From a working tty, I'd start by trying the command:
timeshift --help
or maybe:
sudo timeshift --help
(I'm not sure about whether you'd need to use sudo because I don't use Mint). If you don't know how to use a command from a Linux terminal, that's probably the best place to start.
If you can't get that to work, I'd normally point you to the official documentation, but it looks like
Mint's documentation only covers the GUI. You could also take a look at the
upstream repo that Mint gets the tool from. If none of that works, you may have luck with a web search for something like "timeshift from cli" or "timeshift from tty."
Stuff like this is what drove me away from Debian, Mint, etc. Your distro is likely making some choices on your behalf in the name of simplicity. If that's the case, you could fight with your distribution for control of your bootloader, pick another distribution, or live with it.