Chicken – Why is the meat closer to the bones tastes better

I suspect you may have created a poor-man's slow-cooking environment in there. You had meat, and liquid, and a median temperature of around 200° F, and you probably also got the bird close to "done" during the first broil, before you even left the house. This is obviously easier to do when the meat is covered (was it in a covered roasting pan?) due to the steam, but the oven does provide some insulation to begin with.

Technically when slow-cooking you should theoretically be able to speed up the process by quickly bringing the meat up to just below doneness/moisture-loss temperature (130° F) and then switching to a moisture-preserving slow-cook method like braising. I think that's what you accidentally did, but it's hard to say for because nobody was there for an hour and it sounds like you didn't check the temperature before the second round in the oven.

My guess is that the second roast at 400° F was probably unnecessary, and that the bird was already done, having been cooked in a very slow roast.

I'm pretty sure it wasn't the basting that helped. It seems to be regarded as a myth these days that basting keeps the meat moist, because the baste really doesn't penetrate the skin (and it's not the skin you're worried about). Basting is done to add flavour, not preserve moisture.

The recipe itself also doesn't strike me as anything special in terms of keeping the bird moist, aside from having a relatively short cooking time (as with any grilling/broiling) and letting the meat rest afterward, neither of which really apply in your case. It was probably the slow heat that did it.

Yes, making a demiglace will take a long time. There isn't much of a way around that, as liquid will only evaporate so fast. For a proper demi, you will need to include lots of connective tissue in order to convert collagen to gelatine.

To make a stock, don't boil, it's not necessary. A simmer should be fine. And whatever size pot you're using, you'll want to have approximately 1:1::bones:aromatics, the solids should fill the pot completely, add just enough water to cover.

There isn't that much accuracy required in chopping up the bones, as long as they're relatively around the same size. I wouldn't pan-fry them, I would roast. And yes, you will need a cleaver (or a saw) to cut through the bones; you will destroy your other knives.