Hi,
Regarding the seat, LATAM blocks some seats for airport (and Black signature?) allocation. In the case of narrow-bodies being 1D and 1F. You should be able to get one of these when you check-in.
For the bags I’m not sure, maybe someone else will be able to help
For the frequent flyer number in my experience the website works best but can sometimes not work. I believe that if you add it at the airport it should work. If not you can always claim back after the flight.