Good questions. I'm still confused about the whole thing.
One of the payouts (initiated ~70 days ago) just arrived in full amount (crazy 28USD :X). If the tax wasn't applied, why the 2 month delay?
I also got the email "Are you sure about 30% witholding?" which said I could reply to fill the form again. As my country does have a treaty with the US I thought I might have gotten something wrong and I'll try again. So I replied to that email after a few days, but nothing happened :(
Totally agree though that itch.io is indeed awesome =) Just the tax thing really killed me, in a small way.