Finding this site and reading all these theories from other people = best thing I’ve done this week.
Anyways, I definitely think Rumpel knew who Henry was, and that he came from Emma, and that one day Regina having him would lead Emma to SB.
As for August and the book, it’s been driving me nuts since we got to see him tampering with the book, thinking what he could be doing with it. I’ve been considering the theory that maybe he’s in the book and either took himself out or changed his story. Maybe he even added himself somewhere in there. I’m not sure, just an idea. No matter what he did though I think he definitely knows about the FTL or he would have no reason to show the book or SB in general much interest.