Watanabe shooting himself would no more lead to escape from "limbo" than letting a train run over your head would. The only escape was waking up, which because of the time lag in the dream levels meant only after a very long time. DiCaprio and Cotillard escaped that way the first time.
The hollowness of most of the characters means that any interpretation about the richness of the emotional subtexts is nonsense. It takes long, incredibly detailed posts trying to cover that up.
Watanabe came back to them. Either DiCaprio has near divine powers to predict human behavior (and I didn't believe it when Page came back either,) or the manipulation was kept offstage. That's a story cheat, and worse, a meaningless story cheat. DiCaprio tricking Watanabe isn't a shred more interesting and is less meaningful in an emotional sense than him genuinely rescuing Watanabe. Besides, why would Watanabe project Tom Berenger so that he could confuse him with Tom Hardy in disguise? If you have to entertain complete nonsense to try to find some sort of depth in this movie, I would think that means there isn't any.
The whole suggestion that the final scene is still a dream is cheap and meaningless because it directly contradicts DiCaprio's speech about how Cotillard is just an inadequate copy of the real person. Thesame applies to the children. Suddenly breaking the rules of the game for a "tragic" ending where DiCaprio lapses into accepting a dream as real fails because it is arbitrary but still not tragic, but pathetic. DiCaprio would know that it was a dream. If Watanabe can see carpet and know it was a dream, then DiCaprio can see the children haven't aged and know it was a dream. The audience
has to see the same children in the same clothes so they'll recognize them. Nolan couldn't wait another three or four years to film children the audience might not even recognize.