

You can (and generally do) install gentoo in a Chroot, I recommend using btrfs with subvolumes on your current install, then switching when everything works, that way you can roll back.
But since you’re on arch you probably already know a fair chunk of what is necessary.
- Use flags are more about features, there are other flags like cpu architecture. I don’t recommend you do too much on your first build, except for having some sweeping ones like X and/or Wayland. Then once you have it running you can do a rebuild. I also recommend taking subvolume snapshots before doing major changes.
- Yes. https://wiki.gentoo.org/wiki/OpenRC#Runlevels












I would like to know too. But I guess it really depends on a per app level b/c of libraries, frameworks, and languages/runtimes.