Hi Paul, if you miss the connection through delays outside of your control AA should arrange for you to catch the next connecting flight but it may be worth letting the Air Attendant know if the flight is running late. Regarding Immigration, I suspect you will get off and walk through to connecting flights so you may do that bit at Phoenix. Also your bags will probably be sent straight though to your internal flight, so at least that's one less thing to think about!!
Either way DFW is easy to navigate and there is a sky train that runs around the airport, You also have the added advantage of Craig being there who should be reasonabiliy familiar as this is the airport we use on the stormchase tours
My experience is that US airports are generally well oiled machines so don't think you would have too much to worry about
See you soon,
Claire
Ta
until