Well there is legal ground to arrest him : the use of mercenaries, attack of a sovereign country under false pretences, torture, unlawful arrestation...
My interpretation of that war is that it was conducted to settle old scores, and possibly to enrich the president's entourage, to the detriment of the citizens themselves who will pay thousands of time what the conspirators will earn.
It is proved that they knew they had no ground for an invasion, and seriously they never pretended very hard, which is why there was such opposition everywhere else. Worse, everyone knew it was going to fail.
Nothing illegal with using "mercenaries" which are really more like glorified rent-a-cops in the context of Iraq. I'd struggle to qualify Saddam's regime as a legitimate, sovereign government. He was basically a thug or gangster that killed his way into power and controlled the country and its people as his own personal property. Its like saying its legitimate if the Mafia killed off and replaced your municipal government. False arrests and torture? Its war and the enemy during the occupation wasn't a legitimate army with none of the protections afforded by treaty. We were arresting state-less combatants. How do you handle an enemy who's home state refuses to claim them as citizens? I don't mean it is ok to torture them, but then you have to define what qualifies as torture as well. Its not as clear-cut and simple as all that.
I sort of doubt your last point. Congress approved the hell out of the invasion and surely not everyone there is part of any shady good-ol-boy network as tight as the one you attribute to GWB. I don't doubt plenty of people made money off the war, but an almost majority of the contractors involved in Iraq that won bids were foreign corporations. Sodexo and most of the "mercenary" security yokels hired were not even American. If the war was started just to make a few elites richer, you'd think they wouldn't let the DoD open bid everything.
Also, GWB being some conspiratorial elite Illuminati guy? He was a raging moron, he's probably out clearing brush on his ranch right now.
At worse, I see it as the US government simply deciding it wanted to rid the world of one less dictator, and after 9-11, damn the consequences. It figured it might as well go after Saddam, to settle old scores, as you say and because he has always been a potential threat to Israel.
As for Bush being arrested for crimes, what crimes? A nation has the right to pursue its own self interests and to wage war. Especially against illegitimate reigns like Saddam's and the pretenders that rose up after his downfall.
This is for things like the Guantanamo Bay memos, and such, what by all indications was a top down authorization of government agents violating human rights treaties to which the United States is a signatory. Those things do count for stuff, it's what people like Slobodan Milosevic were jailed on, for the sake of making a legal argument.
Beyond Bush himself, there's also the effective immunity people like Dick Cheney have gotten from foreign prosecution for charges levied against them as private citizens, stemming from his days as a oil drilling CEO. It wasn't the government committing a crime in that case, it's that his position then and now as a major American politician blocking a legitimate legal case.
I'm not all convinced of the legitimacy of foreign nations to levy charges against US citizens. I think the US gov't holds a similar view on this. Now, if the EU had went in and toppled the US gov't arrested Bush and everyone else, propped up an accommodating US gov't and then tried them with the legitimacy and backing of the new US gov't, then it'd be something like a historical, legitimate war-crime mock trial like in the good ol' days.
I guess it does sort of go back to that "they are too powerful to be held accountable." but I also don't believe the kinds of torture or whatever crimes that might have been mandated really amount to shit compared to what other governments do on a routine basis. Does it make it right? No, but I think its a bit backwards to go after Bush and his band of idiots for approving water-boarding on stateless terrorists and then compare them to thugs filling mass graves. Its just priorities.