It has been on unstable since Arch had it. Unstable is just mirroring Arch repos. So it wouldn't give you any idea of when the update will reach stable.
What? I've been running it for a while. Wayland was pretty borked (couldn't figure out how to make it not be 640x480!), but then I have an NVidia card and I believe that's standard
If you reset all theming options to default (icons, window decorations, colors, etc), and backup your config files, there should be no issues upgrading to KDE6. Keep in mind though, most themes for KDE5 don't work for 6 unless the author updated, or you're willing to manually modify a couple files. Most icon packs should be available though.
That's something they didn't handle all too well imo. They seem to be able to detect incompatible stuff, so it would have been quite easy to help you clean it up. Finding all the places things are stored after they just disappeared from Settings was a bit of a pain.
Your question is kind of confusing, KDE does not do anything specifically with Manjaro, I assume that Manjaro users have to wait for upstream (Arch BTW) to mark it as stable, so if you keep your eyes peeled for Arch releases of Plasma, I'm sure you'll have an idea of a timeline.