Hello,
1. Star Aliiance will status match so you'll be instant platinum this year. you just have to mail them your status letter or something along those lines. I've been told they don't even contact AA to confirm so therefore you're plat on both. *correct me if i'm wrong anybody*
2. It looks like United or CO would be your major flyer. With CO offering the most 1 connection flights via EWR. I think United's status gives you more but CO allows you the most freedom in using your perks. Less Blocking on rewards and such and UAL requiring certain fare classes to be considered for an upgrade. But that might all change as CO
3. yes you can get those miles from AC when you sign up. You just request them online or mail in a request with the form they have to claim miles from your December flight.