I'm not sure, both Red Bull and Renault mutually agreed to terminate the contract, so they didn't have a contract for 2016, that was the whole problem (they terminated the Renault one and then couldn't get a replacement).
Either way it wouldn't have been Mercedes. The rules say they will enforce a forced contract on engine suppliers based on those who supply the least first. So with no Renault contract for RBR or STR that left Renault with only 1 customer and Honda with 1.
Actually no, the Renault deal wasn't done until after the season ended. At the point the contract would have been forced (June), Renault had no teams confirmed to be using their engines this year, and no confirmation they would even be in F1. That means both teams would have been forced to use Hondas.
Either way it wouldn't have been Mercedes. The rules say they will enforce a forced contract on engine suppliers based on those who supply the least first. So with no Renault contract for RBR or STR that left Renault with only 1 customer and Honda with 1.
Actually no, the Renault deal wasn't done until after the season ended. At the point the contract would have been forced (June), Renault had no teams confirmed to be using their engines this year, and no confirmation they would even be in F1. That means both teams would have been forced to use Hondas.