While everyone is different, with different airline schedules as well, I find it hard to beat Cozumel for international travel even if I have to do the "bag drag" from the Cancun airport. Just have to be sure to fly into Cancun with plenty of time to catch the ferry over (usually not that hard), and then schedule your flight back late enough to allow for the bag drag -- I usually try to have my flight leaving CUN at 12:30 or later. If you are looking to maximize the number of dives, many dive ops offer afternoon 2-tanks, twilight 2-tanks, and night dives (night dives are great there!). Theres also a dive op who does blackwater dives there fairly often. Another (and cheaper) alternative is to do morning 2-tank boat dives, then do afternoon and/or night shore dives from somewhere like Hotel Barracuda, Tikila, Hotel Cozumel by Wyndham, Blue Angel, etc.
Another option worth considering is SE Florida. I believe Spirit flies to Fort Lauderdale then you'd need to rent a car (pretty unavoidable in SE Florida) -- I'd drive up to the Palm Beach/Riviera Beach area (there is an airport, PBI, there but don't believe Spirit goes there). Then if you plan accordingly with the Blue Heron Bridge (BHB) tide chart you can do morning 2-tank boat dives and then follow up with afternoon/evening/night dives at BHB depending on the time of high tide. Frankly, of the places I've dove so far (see my About on my profile) honestly I'd place SE Florida as 2nd to Cozumel but above the other Caribbean places that I've been. You can find BHB tide charts online at either the Force-E scuba or Pura Vida divers websites, among others.
Forgive me if you already know this, but I just want to mention that once you start adding anything more than a personal item (backpack, etc.) I find that Spirit starts to quickly become less affordable than other airlines -- especially if traveling with your own scuba gear, but they even charge a good bit for a carry on bag. Related to that, I believe Spirit limits checked bags to 40 lbs. rather than the 50 lbs. most other airlines allow, which for me makes it impossible to bring my own gear - even after drying a day my slightly damp gear seems to be around 49 lbs., YMMV.
I can't comment on some of your other areas, but I'll say I haven't heard anything good about diving in Aruba, Jamaica seems a mixed bag of reviews (may want to search ScubaBoard), and for Puerto Rico I've heard you'll need to drive hours West or East from San Juan to find good diving.
