My apologies for never posting "the rest of the story..."
1. Balance the tires.
I went to my local Costco where I get all of my tires (free rotate/balance!) and paid to have the tires balanced. This produced no change in the vibration at high speed/high acceleration.
2. Hello Chevy?
GM's Bolt concierge service immediately disavowed any responsibility. "Oh so sorry...you need to call Michelin." This felt like the start of the "not me" run-around.
3. Hey Michelin Man...
After some effort I finally reached someone at Michelin who was kind enough but said, understandably, that I had to bring the car into an "official" dealer to get it diagnosed. And despite how many Michelin tires Costco sells, Costco didn't seem to count.
4. Return to the Local Chevy Dealer
Michelin pointed me to my local Chevy dealer. They did a "road force balance" test, which I admittedly had not heard of. The service manager said they had a very expensive tire balancing machine (that apparently few shops have because it's $$$) that adds a simulated road force onto the tire as the machine tries to balance it. The machine also gives a rating of how "good" the tire is or is not. And FYI: Getting tires balanced on this machine was >2x a traditional tire shop (e.g. Discount Tire, etc.)
The dealer's conclusion: "You have four bad tires. They all failed the road force test. There must be something wrong with the internal construction." Well, at 28k miles (I think the life of these Michelins is supposed to be 50k, but someone can correct my guess) that's beyond disappointing.
5. Okay Michelin, Now What?
Michelin said they would replace my tires under a pro-rated warranty. What I found really odd about that is the fact that the dealer's report of "remaining tire tread depth" ignored the fact that the tires were not on track to meet the OEM stated mileage warranty. I think Michelin said they'd give me about 35% off of a new set of tires. But the catch was that I had to buy them within two weeks. Now things got interesting.
6. Oh Where Oh Where Can My New Tires Be?
The folks at Michelin first said "...and find a local Michelin dealer to buy your replacement set of tires." Off I went to Costco, Discount Tires, the Chevy dealer, etc. Not one had any tires in stock nor did they have line of sight as to when they would be available. Even Michelin could not give me any answer as to when I would be able to find and buy a replacement set of tires. This set up the ridiculous scenario of "Yes, we will honor the pro-rated warranty" combined with "...you buy the tires within two weeks of now..." and finally "...and the tires are not for sale anywhere over the next two weeks."
I was effectively stuck in a lose/lose situation with Michelin unwilling to actually make good on their issue. It's a pretty sad reflection on their brand.
7. After All That, What Did I Do?
In the end I decided to just live with the vibration. I've since put another 5k miles on the car and tires. And I suspect that I'll run them down to the wear bar indicators. At this point in time I'm hoping someone will have a few recommendations on the lowest rolling resistance tires on the market for Bolts. Please send along your suggestions now!
The good news was that it didn't turn out to be anything wrong with the Bolt itself. No bad wheel bearing, bad CV join, bad axle, etc.
The bad news was the complete lack of tire availability. The Chevy dealer offered me a set of Bridgestones, but of course Michelin was not going to help fund those. I left the experience exasperated and unlikely to buy a set of Michelins in the future. Of course "never say never" as the time will come and I'll have to choose. Until then I'll just keep driving what I have today.