1. Yes, you should be able to check in online 24 hours before departure. You still need to be cognizant of Frontier's baggage check cut-off time and make you arrive to the airport with plenty of time. I would also make sure to arrive early enough to make sure that your passport information is recorded. While I often arrive 45 min before a flight, when flying a ULCC like Frontier I would make sure to be at the airport 90 minutes before, just because they often have much longer lines and aren't known for their flexibility.

2. Only you can tell whether the stretch seats are worth it to you. The seat width is the same, but you'll have 36-38 inches of seat pitch (about as much as many domestic first class products), more padding, and a full size tray table. For $45 on a 4 hour flight, I'd probably do it.

3. If you want some privacy these seats may be a good option. It is worth noting that because they're ahead of the exit row you won't have any recline ( I think the A321s have "pre-reclined" seats anyways, so not sure if that's an issue) and you won't be able to lean against the wall to rest.

Enjoy your trip to Cancun!