This has probably been explain like a gazillion times, so apologies, but...

Does my username get freed up with I add an ENS username? I'd be thrilled to have my ENS name as the primary thing, but someone else running around with the non .eth version seems asking for trouble?

wonder how the dual username standard will play out...
maybe all fnames becoming ens subdomains?

i don't think so

i can switch between alexhook and alexhook.eth in settings

when will we be able to edit/change our fnames?!

I had the exact same question today - thank you for asking 😅 
You should see both usernames on your profile:

Yes, your username becomes available for anyone else to claim afaik

We can always kindly ask @dwr.eth to chime in.