Your conspiracy theory doesn't make a lot of sense. If the US government wanted to hide Iraq's supposed involvement in 9-11 and anthrax letters, then why did it repeatedly claim that Iraq was colluding with Al Qaeda between 2001 and the invasion?

If you posit that before 9/11, the US government worked to prevent people from understanding what was happening, then after 9/11, some claims of a connection are safe, and some are not safe. Flight TWA 800 blew up over New York on July 17, 1996, July 17 being the day of the Baathist revolution in Iraq. Partisans of a cover-up hold that the investigation was stage-managed so as to favor the hypothesis of an accident, and then retired when enough people had stopped paying attention. It would be risky to revive the discussion of that incident if there was a cover-up. Whereas the idea that Iraqi intelligence had a man in Kurdistan in 2001, teaching Kurdish Islamists rudimentary chemical-warfare techniques, is a safe thing to talk about. It implies the possibility of Saddam getting together with al Qaeda, without implying that they already did so.

An enormous number of claims and counter-claims were made about the Iraq/AQ connection. Who made the claims, why, and with what degree of sincerity and plausibility also varies greatly. You had western intelligence sources feeding stories to tabloids, you had regional enemies of Saddam fabricating tall tales. So there was always a lot of noise. Then there are the key documents forming the "narrative of the state", like pre-war statements by the US government to the UN or the Congress, or post-war assessments like the 9/11 Commission report. They offer a history resembling the one you find on Wikipedia.

Then there are a few people offering an alternative history, notably Laurie Mylroie and Jayna Davis. I find that these people have, not just opponents who defend an orthodoxy about al Qaeda (e.g. Peter Bergen), but they have friends from the intelligence agencies who I think act more like handlers.

Thus Mylroie was often advised by James Woolsey, Clinton's first director of intelligence. Mylroie claims that "Ramzi Yousef" and Khalid Sheikh Mohammed, masterminds of the 1993 and 2001 attacks on the World Trade Centre, respectively, were agents of Iraq, working in the jihadist milieu, and she has some evidence for this. However, she then has a contortion of argument according to which these are the names of innocent Kuwaiti guest-workers whose identities were stolen during the Iraqi occupation and used by agents of Iraqi intelligence. Only the real Mylroie fans ever took that argument seriously, and the fact that KSM's cousin Zahid Sheikh Mohammed is known to be part of the jihadi milieu pretty much proves that the theory of the "innocent original KSM" makes no sense. However, Mylroie's theories were first propagated in the early 1990s, before al Qaeda became public knowledge. They seem to me like a half-truth, meant to point the finger for WTC 1993 at Iraq, without revealing too much about the guys who liaised with the mujahideen, because these guys were western assets (against Russia) in the 1980s before they became Iraqi assets in the 1990s.

As for Jayna Davis, her specialty was the Oklahoma City bombing of 1995, and if she had "handlers", they were Patrick Lang and Larry Johnson, two intelligence veterans who for a time endorsed her discovery of Iraqi connections around the bombing, but who proposed that Iran was the state sponsor. Again, that would fit the half-truth template, but this time from the liberal side of deep politics, the one that didn't want a war with Iraq. I'll just point out that in his book Against All Enemies, Richard Clarke (who is almost the definition of consensus reality when it comes to the recent history of anti-American terrorism) manages to mention that Timothy McVeigh's pal Terry Nichols was in the Philippines at the same time that KSM and Ramzi Yousef were working on Operation Bojinka, and even muses in his book about whether Nichols met with Yousef to acquire bombing expertise.

I could go on at great length. And then there's the anthrax letters, which are either an attempt to link 9/11 to the Iraqi WMD issue, or a message from the true sponsor of 9/11 saying we have weaponized anthrax, this is the next stage of escalation, one that can kill tens or hundreds of thousands if it's released in the open, and not just mailed in letters. Contrary to your assertion that the US invaded Iraq "as soon as possible", in fact there was a period of 18 months between 9/11 and the invasion, and only at the very last moment was a concrete deadline provided to Saddam, and only after KSM had been captured and publicly displayed in Pakistan. It is at least consistent with the idea that the US spent those 18 months penetrating and shutting down the command-and-control structure in charge of KSM's part of AQ, before they dared to invade. (And it's interesting that KSM was apprehended at the home of a microbiologist in Pakistan.)

even if Aum was full of "North Korean agents" (evidence?)

I would have to dig up some old notes, but there were a number of ethnic Koreans playing crucial roles - in the sarin release, in the violent side of the cult, in the assassination of Aum's "science minister" after the attack - and there's also a few other connections. There was suspicion on the Japanese right that North Korea was involved. But the very best thing I ever found was a story in a British paper (again, I would have to dig to find it again, but I should be able to do so), on the day before the attack, talking about a North Korean plot to employ BW or CW in Japan. (That's what I remember, details may be a little off.) I consider that story to be the fruit of western signals intelligence, the collaboration of American and British intelligence, and the use of friendly journalists to act as proxies in communication with rogue states. A related example is that a day or so before the 2006 North Korean nuclear tests, there was a story in the Australian media about North Korean nuclear collaboration with Myanmar. The exact mechanics and motivations remain obscure to me, but I think these "leaks" are meant to be seen by the rogue states (who of course monitor what is being said about them). It's a form of signaling.

Don't tell me, North Korea has secretly had the bomb since 1973, right?

The "first Korean nuclear crisis" occurred in 1992-1993. It didn't really resolve until after Kim Il Sung died in mid-1994. That's the northeast-Asian geopolitical context to the Aum sarin attack of 1995 - Communism had collapsed in most places outside Korea, North Korea started diverting plutonium from its reactors, the Great Leader who was supposed to reunite Korea died. The regime would have had plans for the achievement of Korean reunification including military methods and guerrilla attacks in South Korea and Japan. So whether they had a working nuke or just the ingredients for it, I don't know, but the nuclear issue was part of the picture. (It's even been speculated that Pakistan's sixth nuke test in 1998 was in fact a North Korean nuke - a way for them to test a device outside their own borders.)