These are Sandbox invites. In all likelihood your gmail account will become your wave account and the sandbox accounts may or may not stay alive as developer accounts. Spending money on invites is pointless.
No. I've had a wavesandbox.com account since late May or early June. Yesterday in my wavesandbox.com account there was a Wave with a one-time-use link to create a normal Wave account.
These normal wave accounts (@gmail.com) are what everyone is getting now, and are totally separate from the previous @wavesandbox.com accounts. (Yes, one day, probably every Gmail account will be a Wave account - but not for a while).
