The last two seasons should prove to be very Walker-centric. Now that they have been firmly established as the overarching threat their involvement will begin to expand exponentially.
Absolutely. I accepted a long time ago, pretty much when the show started, that there would be several changes from TV to show, and that I'd enjoy it anyway. The creators have shown a tremendous amount of respect for the source material and have made changes that make logistical sense, and not just for the hell of it, optimizing the story for the TV format. There are just way, way too many characters and subplots in Martin's novels, many of which can be streamlined to carry out the same function, something the show has done remarkably well.