Mary did kill the baby, I can see why that's a confusing point in the AU/OU situation, since Mary doesn't really remember her role in it herself and the murders were blamed on Freddy.

But in the end: yes, she was the one to sacrifice the baby. This makes her one of Satan's servants, but since she's suppressed the memory of her bargain, Satan doesn't have as much of a concrete place in Mary's life as he does in her OU. As it all starts to come back to her, that will change, and that's why I really tried to tie the two losses of memory (in AU, of OU) around the same focal point. Which is "What Mary Really Is."

I hope that's a clear enough answer!