Ya i agree that Nagato is stronger than Minato & Kcm is useless against Nagato,but only weakness that we can find against Rinnegan is speed not even Sharingan(Im not saying Shirangan stronger than Rinnegan) can keepup with normal Minato in addition to that Minato has KCM so probably he dnt need any seals to move instantly but only thing Minato can do against Nagato is to dodge his attacks & wound him physically which is too easy for Minato& also we all know that techniques like planetary devastations and other attacks can't hit Minato since he can teleporte some where else,,,,
so Minato win's,,,,,,