I actually think we'll see a bit of shuffling and RedBull will end up with either an official Mercedes engine (i.e. Mercs will drop Will-- sorry, one customer, to replace them with RBR) or a funky rebranded not-officially-X-but-basically-the-same engine. Horner and Marko seem to have burnt "one bridge too far" with Renault, Renault themselves suggested that they would take RedBull as customer only as a last resort and if forced by the rules.
It's a question of Ferrari selling them an engine because they want the money, or Renault selling them an engine because the FIA dictates it.
I'm not sure I can see the logic in Mercedes selling RBR an engine, since they're such a fearsome competitor, and I really doubt they want to break with Williams. Williams is a perfect customer for them: they aren't a real competitor, but they can serve as a place to park drivers from the Mercedes junior program.
