So you're based in DFW? Just call the AA RTW desk in the U.S. They are very good and very helpful. You can always plan online and just call and give them exact dates and flight numbers. If you have at least some segments on AA, I would not expect a problem. If you have no segments on AA, then there is some mixed information out there about whether or not AA will ticket this for you. I'm not sure myself.

Depends upon the airline. I can only speak from personal experience with regard to AA, BA, QF and CX. AA won't issue seats until you have paid for the ticket (takes about a day or two after you book for them to send it to the rate desk and get it back). AA can assign seats on AA, and QF for sure. Not sure about anyone else. For BA flights in First I called the U.S. based BA number. The AA Record locator will be different than the BA/QF record locator, and CX will also have their own record locator. Not sure about other airlines. Be sure to ask AA for all the record locators. For BA flights intra-europe booked into D-class (Club Europe) you cannot assign seats early unless you have BAEC status. For CX flights you can probably assign seats through AA, but sometimes CX seams to block them and you should just call CX. More importantly, however, is that CX doesn't assign in First until 180 days out.

AA will give you a record locator that you can pull up on AA.com with your FF# and pw (you can also pick new seats for AA flights there). If you book with AA, the record in AA online will show all the segments, not just AA. The BA/QF record will allow you to pull up itin on the BA website and on the QF website (and you can pick seats for BA segments in F). No CX access to things.