I actually did a lot of digging into what you are referring too. My digging has brought me to the conclusion that that "leak" of "Twi's Destiny" and Derpy is around 85-90% false. There would be a way too big of a backlash, Twi would no longer be able to associate to the targeted toy demographic, it would shatter the show's lessons and future. As for Derpy, if what that "leak" was true, then why did Hasbro give the go ahead on many of her official productions in toys and mentions in the MLP comic? While her name is still not said, she is still present. There is still a SMALL chance that the "leak" was genuine, but it seems that it is very likely false.