I wanna talk about with you about FNaF 4 staff. (Since you are the best theorist I have seen so far, I want to talk with you)
Now, we understand we are in a stalemate about Toy Girl=Susie. I mean they are the same, it's obvious. But just like you said They come to life at night quote is the problem. If MCI did not happen, this quote won't make any sense. Unless... If Pigtails Girl is talking about the animatronics rather than the children.
If Funtimes were build for keeping the CC at home,(at the night time) Pigtails Girl can say this. And we can understand from the HW trailer, their movements (Funtimes and Nightmares) are nearly the same.Altough I do still think that there are some Security Ani. that exists,(Most likely from old restaurants.)
It sounds crazy, I know. I don't really think this is true too. But I want to get your idea about this topic.
I guess that could work, Im really not sure though. The Pigtails Girl quote just really hints towards the murders we see the Fazbear Animatronics commit in FNAF 1 and 2.
EDIT: I just had a thought.
So, in FNAF 4, Scott didn't have Baby or Susie planned, which means that during FNAF 4, Toy Girl was just a girl, and the Pigtails Girl was CC's sister's spirit (Not a projection made by Circus Baby)
In SL, he decided to make Afton's daughter possess an Animatronic, and so based Baby's design off of Pigtails Girl. So, when SL comes out, the MCI takes place in between 1973 and 1983, leaving the possibility that the MCI may have happened directly before FNAF 4.
In FFPS, Susie is introduced as an MCI child. Scott, deciding that he wanted the MCI to take place during FNAF 4, based her design off of Toy Girl, without thinking about Pigtails Girls quote.
So, I guess it's some sort of accidental retcon or something?
That makes all them equal tho. But as always, Nightmares are hallucinations is proved once again. And even more so, now I believe to you for Elizabeth putting CC back together. In UCN NM Fredb. says Let me put you back together, and pull apart all over again. Sure it's about the endless purgatory, but it can be a refference too. Since Baby is the most similar Funtime Animatronic to NM Fredb.
I'd say that since Shadow Freddy is a phantom, and not a physical entity, Lefty has very little relations to him. Nightmare doesn't even exist, he's a manifestation of Michael's guilt surrounding the Bite Of 83.
So, my take on it is that Henry based Lefty's design off of Michael's Nightmares (which he described in the Logbook), and then gave Smiles And Servos rights to Lefty. Lefty was then grouped with the Rockstars.