2) I cannot answer that since I don't know what kind of backup you have.
3) No, its 100% unnecessary. As long as all the drives except the one you're installing to are physically disconnected until Windows is installed there's no need to do anything else to them.
28 minutes ago, TheGrandMaster said:
Just connect the drive that is in a loop to your other pc and take out your files and then reinstall windows to be completely safe or you can just plug in a bootable drive and try and fix it from there. For 2. yes you can fresh install from your other pc. For 3. Yes back it up and the sata connector is universal it doesnt matter what size of the drive is, it doesnt have to fit in the bay for it to work it can just hang there since you're just backing up.
Why? Thats more work that unplugging 3 sata cables and doing the reinstall in situ. Also the connectors might be the same but the housing might be made to only fit a 2.5" drive.