1. I don't think he will be able to figure out the exact plan, that is just out of his expertise. but he will figure that something is up.And if he discusses with doctor manhatten and rorschach, ozy will be discovered. If he was a character of watchmen, though, as in, using his skills to do the superhero type detective work instead of realistic one, he would be easily able to do so. (And steal all rorschach's glory)
2. No, it seems unlikely that he will let it occur, letting thousands of people die for whatever cause is way too out of character for him. But if he agrees, he could make the watchmen get involved in the plan, by talking smoothly and playing mind games.
3.Ozymandias destroyes him in a fight. As good as a mentalist he is, it doesn't seem he has any kind of fighting experience, he has rarely used gun, at most once or twice iirc, and that too when lisbon's life was threatened, and once because he thought the person was red john. He has shown literally no h2h skills, doesn't seem to have formidable physical strength and tends to get punched, albeit it is his plan to do so to get the suspect into custody. Anyway, knowing ozy, he can potentially one shot the mentalist.