I think you're right about the McCall's marriage being over - but I think it's over from Mama McCall's point of view, and not necessarily Papa McCall's, if that makes sense. So, although I don't think he'd be much of a barrier to her and Sheriff hooking up, I think he would be at least a little bit of a speed bump.

I think Kate being back will draw in Chris, Derek, and Peter tighter to the main plot. I've also heard that S4 will explore Lydia's character a lot more too though, so I'm not sure how that will pay in. But basically, I think Stiles and Kira might be taking a possible backseat (Scott, of course, will remain in the main plot, since he's the main character and Alpha.)