Theory 2

The adults tried recreating Juliette's powers in Warner's mother, but they failed and somehow turned it inside out.

I think this because:
1. Leila's powers manifested when Aaron was a young child, which means she was an adult when it supposedly happened. But for everyone else it seems to have happened in their childhood or young adolescents.
Maybe it happened due to the stress of being abused by her husband and seeing her child being tortured, but I do believe that, just like how Anderson received healing powers, Leila was given skin-hurting powers. Just in the wrong way.

2. We have seen the reverse power thing before, with Anderson.
Sara and Sonya's powers isn't self healing, but somehow Anderson has self healing powers now. He claims that he got them from Sara and Sonya, so that means they must have flipped the healing power somehow. Just like they must have done with Juliette's power and Leila.

Why Leila allowed the RE to do this to her, I don't really know. It was probably Anderson who persuaded/forced her. Or promised that he'd give Aaron some slack if it worked, which it didn't.