I will note (And I always get ignored when I do this, so... Feel free

...
1) Denver was a demilitarized zone. This means they didn't have heavy amounts of troops in the region, just token law enforcement and peacekeeping forces. When Ghostwalker came in, they were woefully unprepared. Plus, the very treaty of the city, with the various zones, prevented most of the countries involved from bringing in additional forces easily.
2) Ghostwalker did not come alone. Ghostwalker is one of the best, if not
the most powerful summoner the 6th world has seen, augmented by his time in the Metaplanes. Ghostwalker had a veritable horde of spirits that came with him.
3) This happened FAST. Ghostwalker came in, kicked ass, and then sought to negotiate all in the span of what, a day or two? By the time anyone really understood what was going on and was able to mobilize, it was over.
4) Note that Ghostwalker swooped in, beat the shit out of the Azzies, kicked them out, and then made a pact with the rest of he powers. Ghostwalker may be a very old fashioned dragon with a very 4th world feudal mindset, but he's not stupid. He made alliances. He bought the CAS by giving them the Azzies sector, and he cut deals with the UCAS and at least one or two of the NAN nations. Enough that when they bellied up to the bargaining table, GW had enough leverage to get a new treaty.
5) What does Denver really have that any of the countries want at this point? Most of them probably said "Fuck it, we'll let the dragon think he's in charge". Ghostwalker didn't prevent any of them from doing business. He basically just said "Hey, I'm in charge. I run things. But you guys can keep managing your sectors like you always did." It wasn't worth fighting over.
Year of the Comet wasn't really explicit about a lot of things, because a lot of it was the initial story, the initial hook. There were a lot of implications behind the scenes. And there was supposed to be a follow up in the next year or two that would have dealt with Denver in more detail. But FASA folded before that project ever got started, and while FanPro picked up the ball and finished the books that were in the pipeline, directions were changed and the followup never happened.
A LOT of thought did go into Ghostwalker and Year of the Comet. And while, yes, the core basis was "WOuldn't this be cool if...", we still grounded it in a certain amount of reality.
You have to blend Coll & fun with realism, but a rule of thumb for us back then was that realism couldn't take away from the fun. This is a world of magic and dragons and giant sprawling megacorps and Amerind uprisings and crazy cyberware. We expected people to be able to suspend their disbelief. And hey, as it always was... If you don't like it, don't use it. It's your game. I've heard of folks who ignored the NAN backstory completely, which I find hard to do because it's so crucial to the entire development of Shadowrun. But... <shrug> There are no game police. And, this *is* just a game.
Bull