To answer some of your questions:
- Fedora has two major releases per year. I’ve only been using it long enough to do one upgrade, but it was basically seamless and the same as any normal incremental update, except it took longer to apply.
- I can’t speak for other DEs, but the Plasma spin provides a system setting to apply updates automatically. I haven’t used it myself, but it’s literally just a radio button so I imagine it’s pretty easy to get working.
- SELinux for the most part is unobtrusive, but it can definitely be a pain when trying to do more advanced things on the system. For instance, it needs to be specially configured to allow systemd-hibernate to work, and I still haven’t gotten hibernate-after-sleep to work at all (though that might not be SELinux’s fault, I haven’t found time to follow up on it. You can also disable it, though, if it gets too much in the way.
I can’t speak to Arc support or RAID specifically, although if the data on the RAID array is vital then you NEED to have at least one backup before you even think about installing a new OS.
My assumption would be that it’s because we don’t really look at mirrors per se but rather the reflection in them, so the definite article is indicating the fungibility of the mirror itself. This total speculation on my part though and I might be totally wrong.