2) Going from 26mm to a 31.8mm clamp diameter is only a difference in circumference of 18mm. Less than 2cms.

I don't think it should be a problem unless your hands are barely wrapping around the bars as they are now.


Edit: That's not taking into consideration the fact that, as I understand, the 31.8 bars are only bigger at the clamp...the rest of the bars are identical at something like 23.8mm diameters.

Edit pt Deux: I think that answers your 3rd question indirectly: Same size brakes for 26 and 31.8.