The idea that i have is that NMM always has been part of Princess Luna. because both represent distinct aspects of the night, wile Luna represents the moon itself and the beauty of the night, Nightmare Moon represents the mysterious of the night and the darkness of the night and because this aspect itself caused her corruption. And the elements only cleansed the corruption of NMM.
Your opinions in this idea?

6460085

Sounds good to me. Luna must maintain balance, or, well, we've seen what happens.

6460174
exactly her NMM part wold be more vulnerable to negative emotions like ones we see that triggered her imbalance so her will need to be very strong to not be affected by these emotions and the time she has transformed far as we know she was a teenager now she is more mature she can handle it better

I see them more as opposed deities who happen to inhabit the same body. Think Selune and Shar from D&D's old FRCS.