From the LiveUSB, make sure to check the boot record, and that Grub is there. If not, look up installing grub properly from a LiveUSB. Here’s a general example, though it’s using Ubuntu (shouldn’t matter much) https://www.fosslinux.com/4477/how-to-repair-the-grub-bootloader-using-a-ubuntu-live-usb-drive.htm

du -hsc /var

Check the sheets to see which directories are taking up your space.

Not impossible you just killed your drive somehow, though unlikely.

Does the laptop have a manual boot menu you can try and select the drive to boot from?

If it still boots off the LiveUSB, plug that in and see if you can view the filesystem of the drive having issues. Double check in a disk manager that it says it’s bootable, then reboot, go to the LiveUSB Grub menu, and see if there is an option to skip booting the LiveUSB and boot from disk. See if anything happens then. It’s only two levels of debugging, but one or the other is going to show if your drive is not cooperating.

100% Brother, but shop from their Refurbished Store and save a ton of money. Comes with the same warranty as if it were new, and everything ships with toner included.

xdg-open is responsible for handling those. You just need to change what it thinks the default might be: https://www.reddit.com/r/linuxquestions/comments/1ha9czj/setting_default_browser_for_opening_links_with/