the partition was still there, but if i tried to boot from it would just kick me back to the bios. unless there's some obscure grub bug that happened to trigger exactly after i booted into windows i guess...

the fix was pretty simple, i just reinstalled grub from a live environment.

Here's a pretty good video about the different packaging formats on linux: https://piped.video/watch?v=ikBPnYwnUMU

What I wish I knew when i first installed a flatpak is that they, by default, do not have access to all the files in your file system. You can change their permissions with an app called flatseal (it should be available in the mint software app). Even then, I would avoid using them for things that need access to system files and libraries, such as IDEs.

All of this looks very complicated. What advantages does this have over other immutable systems, like fedora silverblue or opensuse aeon?

Imo at this point you're better off just nuking everything (after you backed up your important files) and reinstalling with the following process:

• Install windows, with the linux drive removed
• Plug linux drive back and install ubuntu on it (make sure you select the right drive in the installer) If you do this ubuntu should automatically detect windows. If it doesn't you should still be able to boot into either one of them using your device's bios.