First of all, it wasn't implied that Tobi and Madara worked together. Quite clearly, Madara was working with Tobi, there are manga scans that pretty much prove it.
Secondly there is no real proof that Tobi knows Kakashi, and there is especially nothing that proves that Tobi knows Gai.
And I personally don't think its the elder son, but as for Izuna, the possible reason that he could be still be alive is the simple fact that he said he had Hashirama's DNA. That could be used to revitalize and keep him alive longer. And Madara never said Izuna died, TOBI said Izuna died. In the same conversation where he said that the Kyuubi attacking the Leaf was a freak accident.
Also, you ask why Tobi would be interested in a little kids sharingan? If you hadn't noticed, he has quite the large collection of sharingan. Clearly he is trying to collect as many as possible. That includes Obito's.
And for goodness sake, there's a difference between keeping yourself alive for a long period of time (Izuna theorists) and going back in time to do things FARRR beyond his skill level (Obito theorists). The fact of the matter is that NOBODY can go back in time. Obito can't be born, and then go back in time to plan the Moons Eye Plan with Madara as Tobi. It's not possible. Which means that, backed by the manga, Obito is 100% NOT TOBI.
And you say Tobito theorists have been antagonized since the beginning??? There's a reason for that. THEY'RE DEAD WRONG, BUT WON'T ADMIT IT. And it gets on the nerves of the smart people, hence the antagonizing.