Re: If your not on the list, you're not getting in
The UN Security Council didn't authorise the attack in 2003. They only legitimised the occupation afterwards. The legal basis was I think taken from the 1990 authorisation of force, and the fact that Iraq hadn't complied with the 91 ceasefire disarmament agreements.
Which he hadn't. He still had SCUDs.
They were also unarguably in breach of the 91 agreements on chemical weapons as they threw the weapons inspectors out in about 1997. With only about 70-80% of the known stocks destroyed. The inspectors had been crawling over Iraq for so long, that they knew in a lot of cases what equipment and chemicals they'd ordered or made, and so had a pretty good idea how many weapons that led to. They'd not accounted for it all, QED.
I've not read a report that's managed to work out what went wrong, so I don't know exactly what happened. Did Saddam give it away, like he sent most of his air force to Iran to save it in 1990? Or bury it? Iraq's big, but people would know, and senior generals have since joined ISIS and probably have gone and dug it up, so that seems less likely. Or did he destroy it himself? If so why? Why not get the UN to do it, and get out from under sanctions?
As you say, you can't prove a negative, so once they were gone there was no way back. Saddam had pretended to give up the whole program about 5 times in the 90s, then the inspectors had found more than he'd admitted to - and the whole process of search, find, admit happened again.
Hans Blix admiited Iraq weren't cooperating with his inspectors in 2003 - but said he didn't think there were any weapons even though he couldn't know. His credibility was blown because it was him in charge of the IAEA in about 96 who was about to sign off that Iraq had no illicit nuclear program, when the CIA found it, and he subsequently had to demolish it.
Iraq also had the scientists and the knowledge to rebuild their chemical program as soon as sanctions were off.
So it was a mistake to justify the invasion on WMD, because even if Iraq had it, they didn't have the capability to use it effectively (as they'd had in 1990). But sanctions were collapsing, partly because of the collusion of France and Russia, so Iraq would not have stayed contained for much longer - and that was why they went for invasion, because the existing messy containment policy was about to fail.
I think Blair's need to get UN approval was the mistake. He should have let the US do it alone, or had the courage to sell the case for doing it on its merits, not try to sex it up. Iraq wasn't an immediate threat, but could quickly have become so with no sanctions and all its oil revenue.