You forget that Sasuke have to actually active the Sharingan's ability to see chakra as he did when trying to see where Kabuto was, it's not active unless they make it and Sasuke had no reason to have it on when Madara was right there with his eyes close so I don't see what theory he could have been trying, especially saying it was their only shot and saying die.
Madara talking out loud still had nothing about SM or senjutsu in it, only about a person who obtain both Senju and Uchiha power and Sasuke already saw someone like that, Danzo who had Hashirama's DNA which Madara can't do with an Edo so Sasuke more than likely (if her hear that convo since he was more far back) thought Madara had Hashirama's DNA. Sai and Naruto were knocked down and both, including Sasuke, were all behind Madara when the Hashirama's face showed markings of SM so none could see that.
Madara's alive, Sasuke was going for kill shots, they weren't wild shots, Madara evaded them and tried going for a strike. If you look in the pic, you see Madara side step one of Sasuke's slash
You must be registered for see links
and Sasuke's blade is coming from the side so Madara was shifting his body as he side step the slash before. From that point Hashirama's face takes up most of Madara's top chest which is also where his heart is
You must be registered for see links
.
the only thing that I see plausible is him aiming for Hashirama's face to a) get to his heart b) just going for fatal blow c) that spot was there due to how Madara stances was d) your theory could be right