in the comic Veidt played everybody, including all-powerful Manhattan, which is an ending the reader would not likely forsee (I sure didn´t)

also the way Manhattan leaves Earth is so much more interesting and meaningful

yes he is responsible

wikipedia:

"He explains that his plan is to unify the United States and Soviet Union and prevent nuclear war by destroying the world's main cities with exploding energy reactors he had Doctor Manhattan create for him under the pretense of providing free energy for the world. "

(although I can not remember this happening in the movie, DC only?)

which means Manhattan likely is in on the plan

and if this isn´t significant, what is?! Han shooting first? The force not being a biological phenomenon? I don´t think so!