Is it possible that ET planned this entire thing, and specifically picked out TH precisely because he had a young child, and TH was impressionable? Sorry for paranoia but at this point nothing would surprise me.
I think they met on a dating site - plenty of fish. I personally don’t think she picked TH to purposely abuse a child, if anything I think she would’ve preferred a partner without a child. Then all the attention would be on her. It’s this jealousy of Arthur and the attention he got given him that provoked ET in my opinion. But I do think you’re on to something when you say she picked TH precisely because he was impressionable.