It's not getting cancelled. They're wrapping up current story lines and season 7 will focus on an adult Henry.

They did say!
Regina stole the curse from Maleficent who stole it from Rumple who stole it from some cave in season 4 with the help of Ursula and Cruella Devil. they never said who created the curse.

No. According to Adam Horowitz on twitter in the show's universe Hamilton hit Broadway about 18 months earlier than in our universe.