FYI, you don't need btrfs for Timeshift. It has two modes, btrfs and rsync. The latter is perfectly compatible with ext4 and what most people are using (myself included). It just takes more space and a little more time to run. On the whole, though, I've found it easy to use. Meanwhile, to answer a question from earlier, btrfs is still a work-in-progress
and many people think it will never be ready for prime time. That's why I've never bothered with it. Also, probably explains why os-prober/grub haven't been fixed.
Speaking of earlier questions, yes, you can reinstall Grub from a live session. Easily done, in fact. The reason I've not recommended here is that you got in this mess because Cinnamon was having boot problems, so I don't think reinstalling Grub will help. Indeed, I'm not especially confident you'll be able to boot Cinnamon from Kubuntu, even though we've figured out how to get around the os-prober problem. If not, Cinnamon probably can be fixed, but my instincts say this isn't a boot loader problem. Only way to find out, though, is one step at a time.
For future reference, then (not recommending here), this is how you reinstall Grub from a live session. (These are the UEFI steps, BIOS is somewhat simpler.) Set up an internet connection. Open Terminal and run the following commands, one line at a time:
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sudo apt install grub-efi-amd64-signed shim-signed
sudo mount /dev/sda11 /mnt
sudo mount /dev/sda1 /mnt/boot/efi
sudo grub-install /dev/sda --boot-directory=/mnt/boot --uefi-secure-boot
It's the device mount in the second line that determines which system gets put in charge of Grub. Alternatively, you can do the same thing from any bootable Ubuntu or Mint OS in a multi-boot system, except you don't need the first line (those will be installed already).