The short answer is ‘Yes’.
You can add / change / remove flight sectors as many times as you want (for the 6k points fee each time - btw it’s gone up from 5k to 6k...), up until the first flight begins. There are a *few* opportunities to make changes once the travel has started, but the rules are quite tight - Link to Ts&Cs here:
Frequent Flyer Terms and Conditions | Qantas
Regarding the second part of the question, yes, even if the first 3 flights go over the 280k points for J (say you hadn’t used 2 non-QF sectors yet...) then once you come back to the itinerary with subsequent changes that bring the itinerary into compliance with the ‘rules’, the total points cost will drop back to the 280K cap.
Good luck with your booking.