Sorry. I don't think it works like that these days! My speculation is the Merc a package designed from the ground up for everything to work in harmony. Same as the Ferrari. I reckon the days a team sourcing engines elsewhere and proceeding to win..Looong gone. That is why Ron Dennis went courting Honda and ditched Merc.Fact that didn't prove a success is neither here nor there. The premise stands though
That's exactly how it works for every one other than Merc, Ferrari, Red Bull and Renault. But if Mercedes closed Brackley (the premise of this point) they'd still work with their fastest customer to design their engines and that customer would be Red Bull Racing.