In the movie Primer: At first, just as in any timeline, what Abe/Aaron experience is always determined by the past. However, as the boxes are developed and the characters start looping back, it would seem that any events they experience are a direct result from the past as well as any additional influence from the future versions. If this were the case, it would seem that at some point in the future, either Abe or Aaron told Thomas Granger to go back and attempt to modify the future. However, when They find him and realize what happened, they cannot understand what would have caused them to send him back. Did this event modify the future of their current timeline so that they wouldn’t send Granger back, or is it that they just haven’t reached the point in the timeline when sending him back was a good idea? Is it possible that them finding Granger is the event that will eventually lead him to be sent back?
Did this event modify the future of their current timeline so that they wouldn’t send Granger back, or is it that they just haven’t reached the point in the timeline when sending him back was a good idea?
Unfortunately the moment you describe breaks the movie, even when using the movie's own time travel theory.
By getting Tom to travel back in time to disrupt future events, Abe and Aaron stop the very event that forces them to send him back, causing an infinite loop. They will now never reach a time when sending Tom back to disrupt the future seems like a good idea.
1- In the final and definitive timeline (last seconds of the movie), Aaron 2/The Narrator couldn't know about the Granger Incident at all. Granger was the anomaly of Aaron 3's FIRST timeline iteration, the one we watched through most of the movie (foolishly believing Aaron3 was Aaron Prime). So how did he (Aaron 2) know? Mystery.
My solution: the "original" Aaron 2 indeed witnessed the events of Granger. Hidden from Aaron 3 and Abe 1. We're just hearing excerpts of the "original" conversation the "original" Aaron 2 had with Aaron Prime, where the Granger Incident was described and speculated upon. Anyway, in the ABE2-TEMPERED timeline, Aaron 2 never witnessed the events (because they never happened in the first place), so "his" phone conversation must have been different and Granger-free.
2- "RECYCLABLE" doesn't mean "IT CAN BE FOLDED UP" at all. If you read the script, there's a (discarded/deleted) dialogue about the FOLDED UP BOX which means it is a separate concept from Box RECYCLING. By "RECYCLABLE" Aaron 3 meant it can be used over and over to LOOP in the same day (without creating permanent doubles) !!!!!!!!!
Anyway, speaking about the party "re-engineering", Aaron 2 was wrong or just referring to the FIRST Aaron 3, not the ABE2-TEMPERED TIMELINE Aaron 3 we see in the very end. "This" Aaron 3 never looped back over and over. He just went to the party ONE TIME with Abe2, and that's it. The final timeline Aaron 2 was just guessing.
Hope it's clear.
Aaron states early on at the gas station with Abe that he does not believe in paradoxes, and that "things gotta work themselves out somehow right?". So the premise is that you can alter the past and erase your own future without consequence, hence the reason doubles can coexist and only 1 universe/timeline remains. That being said, since Abe and Aaron will never know what Thomas was sent back for until he comes out of his coma, their only recourse to prevent a possible paradox is to simply tell the young Thomas in the present that he simply must go back in time for unknown reasons. Lets say that Aaron realizes paradoxes are possible and they are about to destroy themselves from too much goofing around. To prevent this they simply tell Thomas "we built a time machine, you must use it and disrupt our events at this precise moment in time. You cant ask why, because we dont know why. All we know is that you visited us and disrupted our events for other reasons that we will never know. In order to preserve space-time you simply must go back in time from this point forward for no other reason than to complete the time travel loop, or else all is lost". At that point Thomas could agree and simply go back in time because he is expected to, not because he has any particular mission.