They still ask to present credit card at check in if you buy directly from TG (at least they did last month when I checked in with a ticket purchased from TG)
that the seats are separated could only happen if flight is cancelled and you are moved to another flight, where no adjoining seats are available as it is already pretty full
or if aircraft type is swapped to totally different one, which does not have the seat numbers you hold. (so what aircraft type are you booked on?)
but anyway in those two cases as your two seats are on same record, agent will try to keep seats together in above two cases.
So you certainly do not have to check seat assignment frequently