just my hapence – They had already done so only because they did go back in time. If they would go back in time, they would have had to have saved Harry’s parents.
They only saved Buckbeack because they in the future went back in time, which is obviously the Time Paradox. The only reason why they didn’t save Harry’s parents, Dumbledore and everyone else in the past was because they didn’t decide to. If they would have decided to then automatically they would have in tha past.
Makes no sense. Sort of like “???? ?????? ?????” in R’ Chaim. Oh well.