Correct me if I'm wrong, but did we even know that Tobirama was a **** to Uchiha back then? I don't think so. Also, I think Danzo's idea of the Uchiha came from the fact that they tried organizing a coup on the village, long after Kagami was seemingly dead and gone, and even if that wasn't the case, it still doesn't justify him in the role of Tobi nearly as much as Obito would
Yes we did know that he was a **** to the Uchihas back then. It was in the flashback story about the Uchihas and Itachi. Regardless of the coup introduction or not, it was still mention before the Obito reveal, so my point still stands.
Tobi knew how Madara, Hashirama, and Tobirama was like as well as the details of when the Uchihas started realizing that they were outcast from the village. He knew all these stuff that bridge from hashi/mads era all the way down to the fourth hokage's era, and Obito fitting that quo does not make sense whatever.
Obito only knew so much because of Madara's genjutsu and zetsu's recording, if not for that he would've been left with as much knowledge as Kakashi. That's why kagami seemed more plausible at the time pre-reveal. Tobi had to be someone who was just old enough to know and interact with madara and still lived up to minato's era, and kagami fit that bill until the whole gedo life force thing keeping 80 to 90 year old relics alive.