You're not wrong, Leto going through a heel turn would kind of defeat the purpose of a good ending, however I wouldn't define the revenge ending as a good ending. I know N_Taii is intending to approach the finale differently than he did previously by not having a "true" ending, but if we assume he's still going to have a revenge ending I fully expect it to be completely distinct from the good ending. My hope is for there to be a good ending, in which Leto saves the day, order is restored, his relationships with the girls are restored as well as they can be, and for the duke and company to be punished. An NTR ending in which Leto is betrayed by everyone he holds dear, defeated by his enemies, and the country falls to the oppressive might of Queen Rose and her villainous cohorts. Finally, a revenge ending in which Leto destroys his enemies and wins the day, but the humiliation, betrayal, and manipulation he suffered at the hands of Rose twists his world view.
Leto's lack of a truly defined character makes his endgame status quite malleable and I believe that he should be adapted to the ending themes. If it's the good ending then he's a hero, if it's the bad ending then he's a loser, if it's the revenge ending he's a villain. One of the few advantages of making him a mute character is that he's never going to have strong personality which means it's easy to change him to fit specific situations.