I'm trying to book two one-ways on one ticket in different classes (AABRN).

I want to do F LAS-SJU (via LAX-JFK on 762 redeye - I already have my JFK-LAS booked) and then do return to NYC in Y (S) SJU-MIA-LGA. What's the best way to do this?

1) Book the two one-ways and then call AA to merge them?
2) Book the entire itinerary in F and then call to have them change the SJU-MIA-LGA leg
3) Just call AA to book
4) Is there any way to book this online?

Thanks in advance