Raid is more for an always-on solution, but not great for safe backups. They still might get damaged at the same time
Yes.
I believe it really depends on the amount of data you write to the disks. From my experience: if you’ve two disks, same model, same brand, same powered on hours they might fail at the same time and you end up with nothing thus for most people it might not even be worth to RAID at all on a home NAS. Have a main disk for always online to write / read from and a second disk that is turned on once a day to rsync all data is. Most likely safer and more reliable, you also get extra protection against accidental deletes.