One thing that I keep thinking about is that the tent he had is not available in stores on most of the East coast/Southern states. (see the "store locator" map here:
https://www.brooks-range.com/ ) Someone earlier in this thread said the tent looked like a 2013 model. He most likely had that tent before starting out on the AT (JMO). Since it's an expensive tent, maybe he even bought it long before quitting his job. I know he could have ordered online or got it second hand, but I suspect he has ties to a city where the tent brand is sold or he previously took a hiking vacation someplace where the tent was sold. There is only one store in New York that sells Brooks Range brands and it is in Keene Valley, NY:
Home - The Mountaineer
That store also sells the brand of hiking boots he had:
SALOMON : Running shoes, trail running, hiking, ski and snowboard
If he was from NYC, my theory is that he took a hiking/camping vacation to the Adirondacks sometime around 2013 and while he was still employed. JMO. If we knew what brand his missing backpack was I suspect it would be a brand sold in that same Keene Valley store as well. The other items are less expensive and available in more locations (The trekking poles are sold at Walmart, for example).
Some great ideas, here. That idea about the Brooks Range being associated with a specific store is very clever.
Salomon shoes are sold, like, everywhere. No way to find a clue there. He could even have got them discarded into a "hiker box" (things thru hikers discard and leave at hostels).
The pack is a Gregory. As far as I can tell, a fairly plush Gregory. However, in the photo with the red pack cover, all I can see is the logo, so I can't be sure what model. Gregory's are sold, like, everywhere.
Brooks Range is an online store. There are many tiny companies like this in the ultralight hiking arena. Many are sewing shops in garages. They are geek hobbies. These companies are too small to be carried by regular stores, except maybe one or two stores that I can think of, and even then, those stores might not actually be distributors (they'd instead get the gear sewing company to mail the item direct to the customer; this is called "drop shipping").
A few of these gear makers do well, but the vast majority never make it out of their garages
It's unclear to me whether any of the Brooks Range tents are designed and made in a way that would make them weatherproof or durable. Or whether the quality of fabric is compatible with the price point. A couple that I see online have cheap tarp fabric for floors; this method is typical of VERY low end tents. And there's no parabolic curve at ground level, no bathtub floor, etc. These features have been
de rigueur for at least 40 years.
Our hiker may simply have bought his tent on eBay from a hiker who figured out its substantial disadvantages.
As I look at his tent (a one layer model with almost no ventilation and so small it has to be slithered into), I wonder if he got carbon monoxide poisoning..... He could've tried to cook in it, especially if he'd become ill and couldn't move very well.