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?

i don't think so

i can switch between alexhook and alexhook.eth in settings

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.