Sunday nights and Monday mornings are very very BIG days for business travelers. You should be able to snag an Impala on the EA, if there isn't one, ask the greeter. The Impala has a HUGE trunk (18.5 cf) and is very roomy inside.
Yes, you can go from National to Alamo without a problem. They are right next to each other on the 1st floor of the garage on both A&B side.
Thanks! My Alamo price just dropped, so now it's less than National. I'll cancel the National for now, and if their price improves on Alamo again, I'll switch back.