Very good theory, but I have some doubts and questions. Ok, if i remember correctly, only the "coordinate" has the power to mess with people's memories and also control titans, that's why the first king could erase everyone's minds and how Historia forgot who was her sister( her sister erase her memories) and now Eren has this power but he can't develop that power completely. The theory about the friendship between the Beast titan and Grisha sounds very interesting and could be true, but I believe the plot is more bigger, I don't believe the Beast titan is behind everything. Remember that Reiner and Bertolt always talk about coming "home" and how they need to bring the "coordinate" or they will be punished. This could be brainwashing, like you said, but it's still very interesting.
Now here's my theory, I believe that there is another place where other humans live, humans who know about the history outside the wall and they know how to make titan shifters and they are also enemies of the "normal" titans who just eat humans. They know that the coordinate is inside the walls and thus, they first send Grisha to retrieve it and they somehow erase his memories of the time he spend there so the people inside the wall could not get info about their place. But he takes too long to retrieve the coordinate and they send three more "warriors",Reiner, Bertolt and that one dude that Ymir ate( then she becomes human).
When Carla died, Grisha wanted revenge againts "the others" and he screw up the plan and gave the titan power to Eren. Years go by and they can't capture Eren and thus, they sent another titan, the Beast titan, the one who will do the job.
So yeah, that's mine, sorry for the grammar, I'm typing in my phone xD