Hi CDR01,
Join anyway, is my opinion. Simply write to them saying that you've just booked blah, blah... and that you'd like to join Diamond Club. They'll send you a card anyway.
Do the same for UA Mileage Plus, online though, and then do the maths when you finalise your itinerary or even when you get back.
When you know your itinerary, you'll be able to figure out which ailine's FFP will give you the best award/status retun for your money. Since you won't have status before you leave, you have nothing to lose. Ans some of the finer minds here will be able to help you figure out which FFP will give you the best return.