So we're getting 2 new summoner spells and loosing 2 other spells when we play this mode. So I have a question: What about the masteries?

Will the masteries for Fortify and Teleport work double duty for Garrison and Promote respectively, or will the new spells just not have masteries?
Or maybe Masteries are getting reworked and SS Masteries will be removed completely?

Personally I'm all for the last option. I have having multiple rune pages that just have different SS masteries selected. I could go on why I think this is the best course of action, but this isn't really the place for it.

No.

Where did it say that?

This will probably be a dumb inconsistency that goes ignored forever by Riot.

bump?

They are working on new masteries. They were leaked about 6 months ago and confirmed by a Red. CBF looking for the thread but its out there and they looked pretty nifty.