Not that it means much, but I think that water type did use an ice type move in the video and I actually don't think Nintendo would want you to start with a Pokemon that can defeat your rival's Pokemon with super effective attacks. I think that I agree with you that all of these Pokemon are going to just have a single type. I don't think the fire type is going to gain a second type and the grass type doesn't look like it would be compatible with another type either.
Its not like water types have not been able to use Ice Type moves before without actually having Ice Typing.
As for Single VS Dual Typing I think it is STILL FAR EARLY To tell but I would have to say in the case of Chespin & Fennekin I'd hope for dual Typing. And heck Types could be picked up after evolving.
I wouldn't mind seeing Fennekin as say Fire/Psychic as suggested or heck lets start thinking outside the box a little maybe evolve into a Fire/Steel or something as seemingly incompatible typings are woefully too rare or hell maybe some new Typings like Light which has been suggested since we've had Dark for a while ( Do I hear Fire/Light )
I would have to disagree on Chespin and go with fans Chespin could be any of following possibly ( Even if it comes post evolution ) Grass/Ground , Grass/Rock , Grass/Steel ( though unlikely ), Grass/Fighting who knows though