Ok let me see if I can clear some things up here. First off, I highly doubt that what you would get in the mail would be the actual live pass, since they need to see your photo ID when they issue a pass. I think it would be some kind of voucher that you would still take to the ticket window.
Second, you cannot park your car overnight in the parking garages, especially for days. It will be towed. What you do, and I just did this myself a few weeks ago, is go ahead and park in the hotel parking lot (it is $18 per day plus tax, can't really get around this), then walk (or take the boat) over to one of the parks, get your annual pass, walk back and check in. The only way to get around the hotel parking fee is to not have a car with you, basically just being dropped off and then picked up on checkout day. Just be glad they don't also tack on a resort fee like some properties do.