The ritual is to turn her to the version before betrayal, right? Full timeskip back, turning her younger and losing all her old memories?
At what point exactly, anyway? Just before the first betrayal, so she remembers Sophia? Right after her change of heart, so around the seconds the ABB is going to carve her? Or truly back before summer camp?
My real question is why would we bother? What would we, or even Emma herself, gain at all from doing this?
Suppose it actually does work perfectly with absolutely no side effects. Now what?
She's now a younger version of Emma who remembers having Taylor as her BFF and sister in all but blood. Too bad the Taylor of now is absolutely bat-shit bug-fucking insane and is that way due to an older version of Emma making her that way.
Thankfully she has her loving family to help her through this horrible situation...oh wait....no, older Emma killed them and fed their souls to an evil spirit.
Maybe she's just after meeting Sophia but before she decides to torment Taylor, her other friend will surely help her....nope, Sophia is gone, had the book thrown at her as hard as possible for publicly creating the Fairy Queen 2.0.
There is no point in Emma's life that we can reverse her to that doesn't lead to her having a complete breakdown since her older self saw fit to not only burn but atomize any bridges and support she might have had. Even if I didn't dislike her, that's something that I would hesitate to do to someone I truly hated, which the younger innocent version of Emma we're aiming for is not and would be the one who ends up suffering from the results of our blatant stupidity.
If we absolutely have no choice and the ritual cannot be used on anyone or anything beside Emma, quite honestly I'd say we should aim for regressing her to age 3-5. At least that way she won't have lots of happy memories be torn to shreds by knowing and understanding what she did.