Well, if he isn't then the argument is moot. But we have to trust Rand, and trust context: a famous, controversial architect takes him under his wing and loves his stuff, and the right sorts of people like his stuff. I've found that similar situations occur in real life, where there are particular movements of people with similar beliefs.
If the original plans sucked... well, I think that morally he still has the right. But the point of the novel is that if they sucked he wouldn't BE in that situation. He submits plans because he wants to help the middle class, which will get hurt vastly by this plan. And his plans are good enough that his absolute worst enemy agrees with how practical they are.
Dynamite is the only way to remove architecture, especially if the rights to building it are removed from you. A playwright can choose to remove his play from circulation. An artist can rip apart a painting. Roark can't do that so easily. And I think he has the right to remove his idea even if it's brilliant. In particular, he and Peter signed a contract that Keating would get it built exactly to standards, and Keating broke that. So there's some moral precedent: signs of his anticipating this happening. It's not a random act. Similarly, playwrights can choose to retroactively deny you performing rights if you go to far. It's happened before. There was controversy over Pinter performing a Beckett play a year or two ago, where Pinter changed one of the core parts of the play. It was resolved in Pinter's favor only because Pinter was Beckett's good friend, and understands his work thoroughly.
What's this about the Sydney Opera House? I don't know this story.
And, because I've seen your name pop up before, I feel like I ought to tell you that even though we disagree a lot, I've liked every debate we've gotten into. Thanks. :-)