Update from visiting the Apple Store again just now:

New availability popped up online sometime this afternoon for an AT&T iPhone 6 in 16GB--obviously not what I want, but it was indicative that a shipment had been received by the store. I stopped back by on my way home and the inventory in the store mirrored what was available online--only the 16GB iPhone 6.

I asked the guy (different than this morning) about shipments, and he said the same thing about time (they can be received at anytime) but he did further clarify that they are available for sale after they are scanned in, about an hour or so after they arrive. He also admitted that if they receive a very small quantity--say, 2--of a particular device, it may not show online. He said I could call the store, but again, if there are only a small quantity, they may just say they don't have them (even if they do) in order to avoid having people drive all the way to the store just to find them already gone.

I'll try the first-thing-in-the-morning thing soon, and if I show up at 9:55am only to find two people in front of me in line who will grab the device I want, then you have my permission to say, "I told you so." Otherwise, I'll remain skeptical that showing up at 8am is helpful in any way, since FedEx doesn't deliver in the middle of the night, and phones clearly (based on my experience today) are sold throughout the day as shipments arrive.

I suspect a better strategy will be to set alerts on all the iPhone tracking sites for new availability in any model, which is indicative of a shipment arriving and being scanned in, and then to skedraddle over to the store (it's about 10 minutes from my house) immediately thereafter to see if they have any in my model that may not be showing up online.