It might be easier, but it might not be. My point was that when they tried to go from sedan to coupe, they decided that it compromised the coupe too much for the Mustang’s purposes, so they went a new direction. Going from a coupe to a sedan might also compromise the sedan, so it might be better (if not easier/cheaper) to go from CUV to sedan.
I don’t recall the history of CD6, but isn’t it an evolution of CD4, which was originally a sedan platform?