Interesting both V6's.
I am curious what your car's build date is or it's sequence number (the last six digits of the VIN).
173721
Bad gas? Who's gas and what grade?
Worst I would do is Circle K and Fry's Food-mixed in with Shell, Chevron, QT. Never did a "no name." Since last April and the new engine it has been pretty much strictly QT. Different locations
Arizona; dry, dusty climate down there?
to a degree yes. But my air filter has never looked nasty when removed--always have been surprised how clean it looks.
How often was the engine air cleaner changed?
Was to be checked at each maintenance by the dealer. I changed it last in November.
Almost a 100,000 miles on it; is that mostly city use or highway mile?
Mostly highway. Most areas I travel are 40 mph or higher roads.
Just some random questions that come to mind.
I hope GM makes you happy in the end.
As far as the gas quality goes with your stock vehicle who's gas is way more important than the grade.
Do you always use the same company's gas? QT mainly now.
Shell or Chevron at times.
Here's a couple of examples as to why who's gas can be an issue.
I've have a few of fuel system freeze up experiences in temps below -20C.
Every time it was gas from the same gas station;
mind you each time it was an old dirty filter that should have been change before winter set in;
the freeze up was in the filter each time.
Needless to say I don't fill up at that gas station in the winter any more. "Shell" gas by the way.
A neighbour of mine had to have the entire fuel system on his Volvo replace; gas tank to injectors;
because he made the mistake of filling up at a station that was having their storage tanks replaced.
He was lucky as his insurance company covered the costs.
Now on to that question about your hot dusty climate and the air filter.
That dust will get every where and I mean every where.
Has the passenger compartment air filter every been changer? It should be changed once a year.
Same as above: went to the dealership for each maintenance check. I usually use windows for air unless highway travel, and A/C still isn't used frequently.
Is your interior hard to keep clean? Constantly have to dust the surfaces?
Eh kind of, but is due to windows being down. I like the feel of the wind blowing through my hair 
Well it's late so I'll post this for now more to come tomorrow or the next day.