Wine – How to Know When Wine is Properly Reduced

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.

I have to wonder if you are using a wooden utensil to 'scrape the meat remnants'. If so, stop using wood. The wood will char and leave bits in your sauce, creating the bitterness. Try slowing down the process a bit and using less heat, at least initially as you deglaze the pan. The graininess could be charred bits of food which are burned before they are loosened from the pan surface; this could account for the black spots too.

Cooking is love, and love doesn't hurry.

:)