Why is the stock jelly-like when cold but watery when hot

Some vegetables are aromatics for infusing flavor without changing the underlying base, and certain combinations become known for cuisines. Onion, celery and carrots make mire-poix. Onions, garlic and tomato make sofrito.

Turnips and parsnips are not those kinds of vegetables, usually. Parsnips get very sweet (a good thing usually) as do onions and carrots. You just combined a lot of the ones that get sweet and intensified it by roasting.

If you are looking for a more neutral broth, stay with onions, celery and carrots and simmer, don't roast.

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.