It was merely an option. But I agree with your second point, he can release it and then summon it again. But
it doesn't change the fact that it would still be under Sasuke's Genjutsu.
So Sasuke can simply absorb the Kyuubi's chakra with Preta Path. Nagato has already shown the capacity to absorb huge amounts of chakra. He absorbed the entire v2 cloak off of Bee. Given Sasuke being the initial owner of the eye, and probably having greater ability with it then that particular sick
Edo Nagato, Sasuke can absorb quite a bit of chakra to enhance his PS to a level far beyond that of what Madara can create. And it's definitely not going to be hard for Sasuke to absorb its chakra with his ability to Ameno to it.
Another point, Madara cannot summon the Kyuubi while he's in his PS. He and Obito have both shown to need to put their hands on the floor to summon it [
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]. If Madara ever deactivates to such a level then Sasuke has the opportunity to one shot him with Ameno or PS. Otherwise Madara will have to deal with not having the Kyuubi, while he's fighting a Kyuubi-chakra enhanced Rinnegan Sasuke.
@bold clairfy