I agree with Clatter, I think Rosberg was signed as a replacement for Barrichello. If so, the Button + Schumacher scenario wasn't ever imagined, it was Button + Rosberg. I don't think Brawn considered Schumacher until he realised that there was a real possibility of Button not re-signing and he needed a replacement PDQ.
Ross Brawn was already talking with MS before Nico was even announced at the last race of the season.
It could be that Ross was trying to get rid of Jenson and masterminded this entire move Mercedes definetly prefers MS over Jenson for sure without a doubt since MS and Mercedes have a history together in Group C.