What would it have taken to prove there were no WMDs in Iraq (prior to invasion)?

Among the things that would have helped: a real and honest disarmament program. Done with the cooperation of Iraq. South Africa did it. Libya seems to be doing it now. There are light years of difference between disarmament when the nation is really cooperating, and when the nation is dragging its feet every inch of the way.

If Saddam's regime wasn't psychotic, it still might not have been an issue. Would anyone stay up nights worrying if Sweden got the bomb?

Saddam, for whatever reason (and there have been many theories) behaved exactly like someone who was deliberately concealing WMD from the inspection process. As I've pointed out before, if a cop shoots a known murderer who is holding a gun to a hostage's head, it isn't the cop's fault if that gun turns out to be empty.

That being said, it remains to be seen if the new policy of "War Lite" is going to work. It's never been tried before, and it's dangerously possible that a decisive victory is not something that such an approach can deliver. But that's another thread, I suppose.