Ya the Tobirama = Tobi theory is an old one and I don't think it holds much merit. In regard to your statements about Tobirama's strengths, I think you are underestimating Jounin. I mean ya Tobirama had edo Tensei, but it was very underdeveloped at the time and not to mention, I doubt he had a multitude of ninja prepared at that time for sacrifice. He did not use it like Kabuto is, seems to me it was used by him to speak with the deceased. Maybe he even managed to avoid a sacrifice at the time.
In regard to the Jounin, I really disagree about there needing to be 100+ to defeat a Kage. Think about it, Kakashi's frontline status as a ninja is Jounin. Despite that, he was nearly made a kage himself. Basically, a Jounin may be as strong as a kage. Jounin is simply the highest ranking outside of Anbu and a person isn't an Anbu 24/7. At the very least, a single Jounin has the capability to kill a kage in a perfect scenario so if its 20 on one, those powerful Jounin have at least 20x the probability to gain that essential moment to deal a devastating blow. Basically, I think that they could have killed Tobirama even without the use of the sealing artifacts. Don't forget that all members of Tobirama's group knew it was certain death, meaning 20 jounin v. anyone would mean certain death.
As for Kagami, I doubt he would have willingly betrayed Tobirama. There is no indication he was plotting anything or held any grudge. I think if Kagami is involved, it was done in a second hand manner, like Madara using his body.