I think almost all thing about mitochondria already answered in PE 1, mostly answered by doc in hospital, when Aya and Maya took blood transfusion that doc the one who help them , which is for some reason Maya got sick and need blood . And yes Aya look doesn't age anymore, in PE 2 she look like mid 20 but in PE 3rd anniversary she even look like early 20.

Maya mutate because her body cannot handle the power so it need better body host, which make her look ugly. But Aya physical and mentally better, so she can handle mitochondria power... but if she using her ultimate power her body change for a while to more like and angel.