Adding additional water to already simmering bone stock

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 ask, why would you want to add water? Chicken stock should simmer 8 hours. That is the optimal time to extract all flavours. What you have there is (probably) perfect. However, if you don't want as much flavour, add the water later, not now. Also, to cool, the best way to do it is in an ice bath or a cold water bath (one where there is a constant slow source of running cold water).