Re: Billing order.
Ellen is there more because she's Ellen Page and not because Kitty herself is iconic to the movie audiences. But mix Ellen with the fact Kitty is core to DOFP and her credit is right.
I wouldn't say it means we'll see loads of Rogue. It just means she isn't as tiny as some were believing and she's obviously pivotal to the plot. Plus it shows Singer/producers still see Rogue as a major player in the series. I expect she gets good stuff to do when she is on screen.