The legality of the initial invasion is an open question, without any clearly-defined and universally-accepted answer. Without an unambiguous resolution by the UNSC or ruling by the ICJ, a debate on that topic would fill many pages and might still be no closer to a definitive answer. I definitely don't think it's possible to state outright that it was either legal or illegal.

In any case it's not relevant to the question of whether the current status of US forces in Iraq is legal. That was determined by UNSCR 1483 in conjunction with UNSCR 1511 (sorry, I forgot to mention 1511 and 1790, chalk it up to imperfect memory and a lack of time to research all this again), with the latter resolution explicitly authorising the presence of the Multi-National Force in Iraq as a mandate under Chapter VII of the UN Charter. This mandate was renewed annually until 31 December 2008, when it was replaced by the US-Iraqi SOFA, a treaty between the two countries which established a new baseline legal framework under which US forces are legally permitted to remain in the country.

So there is no occupation in Iraq, let alone an illegal one. If you continue to believe that US forces are occupying Iraq illegally your conclusions and understanding of the situation are going to be flawed and useless. It goes without saying though that accepting the legal basis for the US presence there is not the same thing as condoning it. Though I feel I should point out that I'm not trying to create a justification for the war here, I'm just trying to clarify some of the facts around the conflict. That US troops are in Iraq legally is a fact, it has no morality of its own. It should be enough to disagree with the US's presence without having to wrongly claim that it's illegal.

But for what it's worth, it actually is possible to illegally break into a house and have the legal right to resist eviction, with a number of countries having so-called 'squatters' rights' legislation that protects the rights of people to remain on a property, though usually only for a defined period of time and under certain conditions, even if they occupied it unlawfully in the first place.

As for your last bit, it's an emotional argument which still has no bearing on the specific point you raised earlier. I happen to think Iraq is better off right now than it was either under Saddam's regime or at the height of the war, with their economy improving at an impressive rate, but I'm not sure the bloodshed they went through to get here was worth it.