In the original timeline, Rachel is injured or killed at the party. The first Aaron to emerge from the original failsafe box (hooded Aaron who drugs his "innocent" pre-time-travel self) knows this.
When he is challenged by the later version of himself, he agrees to leave because the newer copy of himself has already done what he intended (record the conversations, then come back again to prevent the incident). So he knows that saving Rachel's life should eventually be successful but not in this timeline.
Before she is killed in this timeline, he explains to Granger that he can save her and how (probably proving to Granger that he has already live through these days using his knowledge of the March Madness sports events).
Granger gives him money as a reward (or agrees to release it when Rachel is saved), and he shows Granger how to go back in the latest failsafe box, with another box inside to re-set as the failsafe box (so that neither Aaron nor Abe will know he came back), so that Granger can follow them around and make sure they do the right things to save Rachel (because Hooded Aaron already knows that even though he's met the later copy of himself, events may play out differently). Aaron is the most money-oriented character, and he knows that he is a quantum-copy and therefore will have to leave his life, so figures he may as well cash in.
Granger goes back to the start of the week and is following the around boys all week (although this isn't seen in the movie). He probably collapses because of the long journey through the box (we saw Abe's fatigue and Granger is older), plus the added stress of worrying about his daughter, plus being awake a lot so he doesn't miss anything (hinted at by the several days' beard growth).
At the end of the movie, Hooded Aaron is starting to build in a much bigger space - this may be a larger box or a facility of multiple boxes. He has said "you won't find me", so he may be setting up multiple failsafes to always allow him to come back to this point and prevent himself being discovered later. Plus he can make it more comfortable, in case he needs to be in there for a long time - e.g. watch the stock market over three months, then travel back and make a real killing (with more chance of avoiding the insider-trading investigation that the original timeline versions would have been hit with if they had placed individual trades on the top-performing stock 5 days in a row), then six months later travel back another three months, etc. [Remember that this version of Aaron hasn't seen the physiological effects the others are starting to suffer].
Or it may be more sinister - know he knows timeline change is possible, he could be planning to use his multi-box facility to give himself the option of going back and tweaking history to his liking. This may involve more interference with his other selves -or at least, the other Abe (the copy from the airport who was going to sabotage the boxes, so the original them wouldn't know time-travel worked meaning the Abe-copy would be the only other person who knew how to build one - perhaps Aaron is making plans to tie up that loose end at a later stage.)
Maybe there's enough potential for an even more confusing sequel.