Is it possible that Itachi knew the truth about T
While it is difficult to say whether Itachi later discovered Tobi was Obito, the light novels Itachi's Story make it clear that in the beginning he truly believed that Tobi was Uchiha Madara.
His reasoning, from pages 53-54:
Over the last three years, Itachi had wondered any number of times about the man's true identity. From what he could infer, Itachi had thought the man was perhaps a member of the Uchiha clan, but that changed to a certainty now.
[...]
When he thought about it like this, the possibilities naturally narrowed. The first was that a member of the Uchiha clan thought to have died in the Great War had survived, and become the man in the mask. It was a general rule that if a member of the Uchiha clan died in battle, their sharingan would be brought home by another ninja. So, the man couldn't have stolen it from a corpse. In which case, it made sense to assume that someone thought to have died was alive, and coming to the compound with his sharingan still in his possession.
The second possibility was that the man in the mask was someone who had broken away from the Uchiha. This was less likely than the first option. Because since the founding of Konoha, only one man had ever broken away from the clan and left the village.
Uchiha Madara...
And Itachi does end up going down the path of accepting Uchiha Madara is alive, and concluding the masked man is him, due to the overwhelming chakra he sensed, and the fact that Madara's death was never confirmed. I have also noted in the text, a part where Itachi was actually going down the right path with his thinking, one which might have led him towards Obito. Due to this, it is possible Itachi figured it out later, given his time in the Akatsuki and proximity to Toby.