About the differences in the resetting system between the games, here's what I know. If I remember correctly in Melee whether a move will reset or not it depends on a percentage threshold, I believe it is up to 9%. Moves that do more damage than that will never reset and moves that do less will always reset, even at 999%.
In PM it is based in a knockback value threshold which I don't recall the exact number, this means that many moves will reset at very low percents. If a certain weak move has enough knockback growth to surpass that KB value it means it won't reset forever. So only a move with fixed low KB will be able to reset even at 999%.
As for why two hit resets aren't possible or why non neutral forced getup resets seem more common in PM I unfortunately do not hold enough information to answer. I believe we might need more investigation on the matter, but it's likely that Magus knows what causes it.