"According to the UN, every country is sovereign and it is forbidden to interfere in internal matters of sovereign states.
The WMD pretext is a way to prove Iraq could be a threat to its neighbouring countries, and because the threat precisely means interefering with internal matters of neighbouring countries, the UN allows the use of the WMD's argument as a reason to prevent such a threat by attacking that country (if the threat was substantiated and more than just potential).
Here, not only was Iraq no threat to its neighbours (since 1991, it hadn't attacked any other country) but it apparently didn't even have the weapons to be a "potential" threat."

You are so dense, it's utterly amazing. How would Iraq do some conventional attack of it's neighbours, when it doesn't even control it's airspace?
The "WMD pretext" is only significant because it was part of the '91 cease fire agreement. That fact that Iraq would threaten it's neighbours [or other countries in the world] if the Coalition forces did not do something to prevent it, was the reason given by coalition leaders to convince their public to support spending blood and treasure on this matter. That some country did or did not follow some detail of a cease fire agreement is hardly a good reason that would convince the public to spend blood and treasure. If you instead focused on that as "THE REASON" then the general public might think that just changing the cease fire agreement or simply not care in the least about it.
Part of any cease fire or peace treaty, are usually stuff like "prove to me you won't attack again, as soon as I withdraw forces or I give you time to regroup your forces". Or "I will stop attacking you now, if you will agree to end the war". So besides achieving a specific objective: Having one less country having WMD. The other "unspoken part" is that if Iraq did this, that proves to some extent that Iraq at least wants to try to show to other countries it has peaceful intentions.

It's not like we had some burning desire to maintain a "No fly Zone" over Iraq. We didn't spend billions per year, because we lacked some desert to fly over. We did this because Saddam provided no evidence that his intentions were peaceful AND quite the contrary, openly declared his hostility. The simple fact is that it was within Saddam's power to end the war at any time after '91 up to the point the coalition invaded his country and remove him from power. He NEVER took that option & he probably die some miserable death as one of the final results of this stupidity.