Each previous generation has introduced no fewer than 100 new Pokémon, so I think it's safe to say we'll get to at least 749.
You're probably right about how transfer will work, although it wouldn't surprise me if X and Y were released as downloads as well as carts (like some of the 3DS games have been already).
I bet it's either the same or extremely similar, unless they drastically change game mechanics. Don't forget, they still have some room to play with in the available data.
I'm sure it is.
I doubt it's any of those. Rather, they don't want to reveal additional new Pokémon yet, so they're showing off the starters in battle; it's simply for demonstration purposes.
Either that, or it's disabled for the purpose of this trailer.