Nor am I perfectly convinced that Ozymandias' projections were correct in the first place. The protagonist of the pirate comic becomes a monster from the desire to save his family from a nonexistent threat. In the real world, only the US government has used or made it official policy to initiate use of nuclear weapons. In Watchmen's fictional universe, Tricky Dick is riding high at home. Why would he queer the deal?
Note that Ozymandias isn't the only person assuming that the strategic situation was leading to catastrophe; the excerpt from "Super-Powers and the Superpowers" makes this clear, and foreshadows Ozymandias' plan. Granted, the reasoning doesn't make all that much sense. A lot of talk in the 80s about nuclear weapons also made no sense. I'm sure you remember all the nonsense about the nuclear freeze and cruise missiles.
Edit: I've got no idea if Moore meant the threat to be real or not. The unreal threat from the pirate comic might not be an intentional parallel with the main plot situation.
One last thing---I don't see how Silk Spectre and Nite Owl II could have avoided thinking they could suffer Rorschach's fate.
Meaning that Ozzy would kill them? I don't see it. He hadn't demonstrated any such intention before they agree to go along with his plan. Is he supposed to change his mind later?
As for the ending, Moore said that he preferred to leave it in an indeterminate state, but that given where the name Ozymandias comes from, it's highly likely that all Veidt's works and plans would come to nothing. For whatever authorial intent is worth.