Re: Madara's EMS powers locked away into his normal 3-tomoe sharingan
It's an interesting idea. I've always wondered if the cost of imperfect Izanagi with the EMS might only be a reversion to a three tomoe Sharingan, which may may have happened at the Valley of End fight. However, I'm not sure how that fits in with his space-time ninjutsu, if it's a MS doujutsu.
Madara's space-time jutsu may be centered around his eye for 'style' to emphasize his Uchiha clan trademark Sharingan (he may be able to center it anywhere), not necessarily because it's a doujutsu, albeit that's most likely at this point. As we've seen, the Sharingan is both functionally and stylishly important to him.
As well, with your theory, it's notable that the Senju and Uzumaki were on very good terms during Hashirama's time, so possible he may have learned or more likely adapted a seal from them. He was even married to a powerful Uzumaki, Mito.