A good job with prospects in the majors in the US? Don't do it. However to answer your questions.......
How often can I plan on being in the USA?
As a junior guy, say 3-4 times a year for 24/48hrs plus vacation.
* How much money can I realistically plan to save per year, living a modest lifestyle in Dubai?
As an FO with a family, not a lot. Dubai is expensive. As a single guy, a little more but not heaps
* Do I stand a chance at seeing a Captain upgrade?
5-7 years
* Do I have any say on whether I go to the B777 or the A380?
Nope, you get told.