

are rolling release distros stable enough
I’ve been using Arch (or some flavor of it) for several years, and I’ve never had any serious issues that I didn’t cause myself. The thing that might catch you with your pants down is if a dependency introduces a change that breaks another application, but catastrophic failures are fairly rare, as long as you’re willing to learn how to maintain your system.
is it possible/easy to roll back to a previous version
Yes. The application is called Timeshift, and it’s specifically designed to back up the system files to a separate partition. If your root filesystem is btrfs, it can also manage filesystem-level snapshots that you can roll back if you bugger the system.
Gparted. I’m pretty sure it’s bundled with Mint. Identify which partition it is, then simply delete it. Then you can create a new partition on the newly unallocated area or extend an existing partition onto it.
You might have to edit the bootloader’s boot entries and remove the Windows boot manager.