Timeshift is a system snapshot tool, not a backup tool. Do not use it for backup, or it will bite you in the butt. The official Timeshift documentation is pretty clear about that, it explicitly warns people against using Timeshift for making backups.
If you want backup, consider this:
Good idea. Try running LTS, if everything works, no need to use the mainline one (but keep it installed, just in case).
If you like Micro$oft having your dotfiles, sure. Personally, I see no point in doing this.
Good idea. It doesn’t have to be the latest one, just one you can boot from and chroot if necessary.