Now you're seeing what i'm seeing, but the part you're missing, is when tobi said itachi found him and gave him a ultimatum.Here's the facts
Sasuke just completed his first semester at the academy, that's less than a yearright? Then he got his report card, his Dad was indifferent
That night Itachi and his dad were talking abt the whole pipeline stuff and a meeting the next day
The next day Itachi and Shisui missed the meeting and went on a secret mission
The day after the meeting, Sasuke showed Itachi the report then Itachi told him the whole I am a wall...blah,blah,blah then those Uchiha idiots came to ask Itachi about the meeting the previous day. They said within half a year of joining ANBU his speech was getting strange
CONCLUSION- he'd been in ANBU just six months and this is proven by the fact that Sasuke just completed a semester in academy. We know Sasuke entered Academy when Itachi entered ANBU since his Academy entrance ceremony was the same day of Itachi's mission that promoted him to A
NBU.
More Facts-
Itachi and his father weren't speaking. Their father passed Itachi and met Sasuke and asked him about classes. He said he was best then his father took him to the river and showed him the fireball tech, he couldn't do it and then trained(for a week according to databook) and showed his father. His father was proud of him. Sasuke then voiced his concern that he was just a replacement of his brother to his mother a few days after his father commended him then his father came in the room and he asked his father about Mangekyo Sharingan then went to school and came back to find his clan dead.
CONCLUSION-Sasuke's father asked him about classes meaning he had entered another semester. It would be safe to say it was a year or more since Sasuke entered the Academy(also the amount of time Itachi has been in ANBU since it was around the same time Itachi joined ANBU that Sasuke entered Academy) he trained the fireball tech for a week since his father asked about classes and mastered and showed it to his father, his mouth was injured from the training. This makes it still a little over a year since he entered Academy and Itachi joined ANBU since its only a week difference. Itachi massacred the clan at above 13, since according to databook there's a 5 year difference between Sasuke and Itachi, Sasuke was 8 when the clan was massacred. Databook stated that Sasuke was 7 when he mastered the Fireball meaning that a year passed since Sasuke mastered the fireball, at the time Sasuke Mastered the Fireball, it was a little over a year since he entered Academy, at the time of the massacre another year was added to that meaning that Sasuke had spent two years in the academy when Itachi massacred the clan.
Itachi had also spent two years in ANBU since he joined around the time Sasuke entered Academy. If he spent two years that means he was 11 when he joined ANBU, this makes sense as Sasuke mastered the Fireball tech when he was just about a year as an academy student making him 6 years old when he entered Academy and Itachi 11 years old(5 year diff) when he joined ANBU. Itachi at the time of Shisui's death was already a double agent and was considering an Uchiha wipeout since he said he has 'lost all hope for the clan' He hadn't been given the mission but he had come to that conclusion. This was because of the promise made to Shisui. For the sake of peace he allowed Saru attempt peace talks for the next year, his father knowing that Itachi had betrayed them since the village knew about the plan, he lost all hope in Itachi and started training Sasuke. Itachi was 12 at the time Sasuke mastered the Fireball, he hadn't met Tobi yet. We know he was promoted to ANBU commander before he was given the mission. The village head gave him the dangerous and heartless mission believing it was on his level since he was an ANBU commander. How does a 13 year old become ANBU commander? The answer is Mangekyo Sharingan, Itachi awakened it at age 11 so till the massacre he had two years to maaster it which is why Danzo knows about his Amaterasu and Tsukuyomi, and probably why they saw him fit to be able to wipeout the clan alone at 13. Itachi said he was trained by Tobi so I believe Tobi taught him how to use Tsukuyomi and Amaterasu meaning he met Tobi before he turned 13 and became ANBU commander. He met Tobi at 12+.
ULTIMATE CONCLUSION----
Joined ANBU at 11.
Awakened mangekyo still 11.
Met Tobi at 12. (If he ever met Oro, it was at this point)
Trained with Tobi till he was 13.
Promoted to ANBU commander at 13.
Massacred the clan at 13.
Joined Akatsuki at 13.
Tobi would Have to do what he says or there would be repurcussions....we know 1 of the elements of there deal, was that iakatsuki could not attack konoha as long as itaachi was dead.... And tobi honored that.... why do you think tobi began his plans right after Itachi died?, So when he met, he must have known tobi was the leader of akatsuki, he could have found out about akatsuki from danzo who saw nagato that day..A final demand or statement of terms, the rejection of which will result in retaliation or a breakdown in relations.
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"You are yuhiko of akatsuki! you should be dead!"
"Danzou who allied with had betrayed us"
So itachi must have been giving some mission to spy on akatsuki and nagato, danzo possibly didn't know anything about tobi at that point, just knew that there was a group called akatsuki, and 1 of there members had rinnegan....considering them a threat , he sent itachi...There is where itachi discovered the rinnegan person was not the leader, and found tobi....... then the ultimatum was given.....Which i believe went like this, 1) Train me, 2) help me kill my clan , 3) put me in akatsuki as a false member..
Tobi agreed and talked to pain...as you may have known pain follows tobi like a btch, and would have let him enter.....Whenever that was orochimaru attacked him, failed , fled..... etc....Itachi had been relaying akatsuki info to danzo the entire time, untill he was 13 that is...which is why at the kage summit he said "I can't reveal my source"