I see it the way Taco John describes it. The top wobbles but then spins freely at the end - it's a dream. It's no more complicated than that.
His kids look the way they do because it's his last memory of them. They might even be wearing the same clothes. Not coincidence. He then proceeds to fabricate the rest (them turning around) as his visit to limbo progresses.
Prior to that Saito and Cobb are both still attached to the machine, so it makes sense that their subconscious can interact with one another.
It's important to note that at the end Saito cannot kill Cobb and then himself while under deep sedation and hope to move a level towards reality. Dying in this state means a fast trip to limbo (which is where they already are apparently). To get out, one needs a kick from the realm closer to reality (or for the machine to be turned off) - and these guys died / missed the kicks. But Cobb is dreaming freely in limbo - so his way out is to forget the rules and generate a means to an end (his children).
Yes, the two of them can be woken up, sure, but Cobb's limbo will last a long time. An earlier remark or two in the film indicates that anyone landing in limbo through its full duration under heavy sedation will have their brains fried.
Cobb's limbo dream world continues past Mal, past the visit to Saito and onto his kids.
In a movie of complicated dream worlds, Christopher Nolan would have missed a grand opportunity to end the film in the way he did had he applied the alternative, rosy finale that saw Cobb to his children in complete reality.