There are still several ways for him to survive. You mentioned two of them.
Obito could still regain control and do rinne tensei before losing his last eye. Also Obito is the only one among them, that can put up a fight versus Madara.
Gedo Mazou arms could be sealed inside Naruto, we dno the effect though.
Minato takes back the Kuybi from dark Zetsu. He is Minato after all, and who knows what plot hax we are about to see. If Gai sensei reaches the scene, he will probably use 8th gate to fight Madara, we don't know the capacities of the 8th gate, but if it could restrain him for a while, Minato/Kakashi/Lee could take back the Kuybi and maybe even destroy black zetsu.
Of course the most likely option, is that Naruto has the part of Kurama from the sage tools sealed inside him. But really, none of the shinobi at the field (except Obito) would know how to use them I guess?
However, there is also another one, if TenTen sees Madara she can just solo him negative diff, U_U