who the hell said nagato can do both at the same time he does not have unlmited chakra he gets tired by just using gedo mazo and even gets hurt badly and he can just use Ct on the side so easily which all requires time where as nine tails can actually form its attacks very fast and can crush the very ground he is in with large scale tbbs.If Kyubi could destroy C/T before Nagato can summon, he would have raped Bunta in a sec. C/T>weight of Bunta.
And on your second point, lets say he summons before he does C/T . Whatever floats your boat.
Hell kyubi doesnt even need to be caught in C/T. He could just have his summons distract kyubi, summon Gedo and rip his soul out.
Tailed beasts are not known for their brains you know.