The best bet is usually to check with the airline that you are flying on. They have up to date information. You can't go wrong if you carry your US passport. Failure to have the correct documentation will cause the airlines to forbid you to board. And you probably will not be able to get a refund on your tickets if this happens.
Travel agents can no longer issue boarding passes.
Everybody has to check in at the ticket counter for their boarding passes. This is a part of the FAA security guidelines. Most airlines are going to a one bag carry on limit. You'll just have to check more luggage if you travel with a lot of stuff. Also, that one bag has to fit in the overhead compartment.