There is one thing that PROVES that the last scene he was still in a dream and he was in limbo - the kids. They are in the same position, same clothing, and same age as the last time he saw them and had been dreaming of ever since (explained previously on the forum).
He saw their faces and thus accepted, in his subconscious or consciously that this limbo world was the one he knew he'd be the happiest in without the guilt of his wife's death (just previously resolved).
What I don't understand though is that Mal was beginning to believe that their world was a reality when they were in limbo together for all those years. And used her totem that she locked away to show that it does fall and she believes that she's in reality. But Leo knows it isn't and spins it and manipulates it to spin indefinitely, showing her that it is indeed a dream - i.e. his first inception. This in turn broke her into thinking the reality was also a dream, regardless of whether or not the totem (her totem, and then later his) stopped spinning.
So they killed themselves to get out of limbo right? So killing yourself does get you out of limbo while you're in it, but killing yourself in a deep layer of subconsciousness (achieved by being sedated) puts you into it. We know what they were experiencing together wasn't a layer of subconsciousness but limbo because Cobb actually said it was. So I know that shouldn't be questioned. However, were their limbo's different? That is, did Cobb and Mal drug themselves before going to sleep and eventually into limbo together (which is what Cobb did call it, it wasn't a layer of subconsciousness)? Or were they only drugged during the heist, thus making going out of limbo impossible until the drug worked off? (unlike Cobb's and Mal's limbo)
Therefore, if Saito killed them both after Leo convinced him in their conjoined state of limbo that it isn't real, wouldn't that have woken them up and not sent them into an even deeper limbo? (based off of how Leo and Mal woke up the last time from limbo by killing themselves)
So if the whole "kids are the same" conundrum almost proving the last scene was a dream, how did he get there? I understand he got into a limbo with the older Saito by dying in the collapsing fourth layer (Saito died in the third, but they still met in limbo as they were still connected, sharing Fischer's dream, just Saito was much older because he had been there much longer). But how did he get from limbo with Saito to where he was in the final scene, again going with the theory that the final scene was a dream?
However, if Cobb and Mal didn't drug themselves, then the rules are changed and that while Cobb and Mal got out of limbo by killing themselves, Saito and Cobb killing themselves wouldn't wake them up, but only send them deeper into limbo because they were indeed sedated. Therefore, the last scene was Cobb in deeper state of limbo as a result of Saito killing him when they were already both in limbo, which would answer my question and prove my theory completely. But I can't figure out what the true rules of limbo are. What do you guys think?