Chicken – How to reduce the masala taste from chicken roast

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.

Roasting the bones will give you a darker brown stock than using the raw bones. To roast the bones, just stick them in an oven on high heat, around 450 for about 45 minutes, or until they are a nice golden caramelized color. Though you will want to make sure to keep an eye on them the first time, I'd check every 5 minutes after half an hour. Roasting the veggies with the bones will also add a slightly sweeter roasted flavor. It's like the difference of putting slices of raw onions on a burger compared to caramelized onions. You get a slightly sweeter, richer roasted flavor. Though it does turn down some of the other flavor notes, it's up to you which you'd rather have. Without roasting, you'd have a clearer "white" stock.

If you are using the butter as a base to roast the bones, I would set it aside and go with a higher smoke point oil like a peanut or corn oil. The low smoke point of the butter could leave a bitter, slightly burned flavor, especially with the longer roasting times.

I would leave the seasoning for the stock. Most spices will burn at a lower temperature, and salting the bones before you make your stock, could make your stock overly salty. It's easier to add more toward the end, than try to figure out what to do with salty stock.

I hope that helps!