Oh, I can weigh in on co-op a little, finally. Got a chance to play it with my cousins last night. Basically, it's alright.
Having only two players makes it significantly less madcap than four player NSMB Wii, so despite the smaller (and single) field of view, we didn't have much trouble staying together when we wanted to.
Of course, at other times we tried playing it a bit more competitively, with frequent butt stomping and rushing on ahead. This was mildly fun in its own way, although the game definitely plays better when you're not trying to screw over your buddy.
I remain of the opinion that splitting the two players' screens up is not the magical solution that everyone thinks it is. This would encourage you to not "play nice", and the person in the lead would have all the fun, while the lagging player suffers through an already-played level. This is, quite simply, not good design.
I fear that overall NSMB2 is just not a great fit for co-operative play; there is no good alternative, barring creating special two player levels which include obstacles that need to be passed in pairs. If the two players do work together, it's not a bad experience, but it really doesn't enhance it in any way.