I think your theory is a creative one. It was a good read, however, the main component of the theory seems to be fantasy, rather than that it is build around a a base of strong facts. Maybe you really want the mystery person to be Obito. I dont know if that has something to do with it as well.
Obito said in his mind, how he couldnt use Kamui, because the Juubi was inside of him.
That seems to be the reason why he didnt use Kamui.
That means he probably couldnt have made the second version of himself, go to the Kamui dimension to hide.
Which means sensors like Naruto and Hashirama, would have sensed the second Obito.
The moment Madara became got hold of Hashirama's sage mode, he could have sensed the second Obito as well.
I bet Juugo would have sensed him as well.
Apparently Karin, can sense at least part of this mystery person, even though the person seems to be in sage mode.
I would just find it really weird if it is Obito and that somehow nobody sensed his presence all this time and
that nobody sensed him making a second version of himself.
Kishi would have to come up with a really good explanation for that.
Its the main reason as well, why I believe your theory wont come true.
I enjoyed reading it though.
Even if the second Obito, could somehow hide in the Kamui dimension, than I would find it
weird how this second Obito, can use sage jutsu, if he doesnt have the Juubi inside him.
What I would also need an explanation on, is how Kakashi didnt notice Obito.
Kakashi has been in Kamui while Obito fought with Naruto and Sasuke.
Tbh, I just believe the mystery person to be Kabuto.
Kabuto's story isnt finished, so it would be a great opportunity to continue his story.