Nice theory & +rep for the effort. However I inclined to disagree for the following reasons:
1.)Kabuto believed that IWR was the greatest jutsu ever created in NU. He considered the jutsu "almost perfect" as if it's free from any weaknesses or risks even with the caster. Thus, he was so confident that he will gain control of the war & no one can stop him from succeeding so. This is one of the reasons why he kept praising Itachi & stated that he was on a different level as compared to the others because he broke out from his Edo-tensei. My point here is, Kabuto wasn't aware or didn't actually know that even Uchiha Madara can free himself from the said jutsu.
2.)Madara may or may not know Orochimaru. But regardless, what will he benefit from him? Madara clarified that he created his power. Therefore, he awakened the rinnegan, & implanted Hashirama’s cell within him all by himself. Possible reasons why he knew the seal was because he must’ve seen it performed, & perhaps this is the reason why he knew the very purpose of that jutsu. Needless to say, he possess sharingan, thus, he can copy non-kg jutsu anytime he spot one.
3.)In my honest opinion, the reason why Sasuke assumed Orochimaru is still alive is because Edo-Madara stayed. As Jugo stated, even though Itachi & Sasuke stopped Kabuto’s Edo-tensei, Edo-Madara is still around. Sasuke was shocked & said, so, he is still around. It seems he was talking about Orochimaru this time. Then he had a background memory of Itachi & Shisui with lines of protecting the village & the Uchiha names or whatsoever. At any rate, I think that the very reason why Sasuke needed to meet up with Orochimaru or concluded that he was still alive was because he thought Orochimaru was the one who revived Uchiha Madara. This will be his own assumption as to why Edo-Madara did not vanished as compared to his brother. Sasuke wouldn’t know that Madara knew the seal or he could break through the Edo-tensei jutsu. Thus, he’ll be going for Orochimaru to certainly stop Edo-Madara for the sake of Itachi & Shisui’s name.
4.)Lastly, when Kabuto answered Madara that he is “his assistant”, I’m pretty sure he was referring to Tobi & not to Orochimaru. Madara asked him if he knew about their plan, (the Infinite Tsukoyomi) then Kabuto replied, not in detailed. He eventually said that he’ll be on Madara’s side than be on the fake Madara. The Moon’s Eye Plan were the ultimate goal of both Tobi & Madara, it’s still unknown if Orochimaru had the knowledge on the said ambitions of the two.
Edit: I somehow agree with your assumption that Madara has Orochimaru’s cursed mark on his neck. It is highly probable & seems more entertaining if eventually he took over Madara’s body.
