I completely disagree with the searing method. You will never get an even color on the bones and will probably have some burning which will give your stock a bitterness. Also, you waste time tending to the bones instead of cutting your veg while they roast, which is obviously more time efficient.
Where I work, we do no roast or blanch the bones because that's the way the chef wants it done. I keep my mouth shut. The bones should either be roasted or blanched. Why? To remove surface impurities which will give you a clearer stock. The choice to roast or blanch will depend on the end. A roasted bones will give a darker and more complex tasting stock thanks to the Maillard reaction. Blanched bones are good for a white veal stock. My sous-chef claims that roasting the bones yields a bitter taste...to which I replied, ''only if they're burnt".
As for the hot/cold start debate, cold is the only way to go. "A hot start produces many separate protein particles that remain suspended and cloud the stock" - Harold McGee On food and Cooking. A cold start will allow all the free particles to rise to the surface which can be skimmed off. It is also equally important never to cover a stock because it prevents boiling(which will cloud the stock) and also allows the surface particles to dry out making it easier to skim. Adding ice cubes to a stock has a similar clarifying effect. To make it easier to skim, you can put the pot only half way on the burner so the scum will tend to gather only on one side of the pot. I would also advise to add the vegetables only after skimming a few times, it will make it way easier. I read somewhere that to have the clearest stock possible, it should be brought to a simmer as slowly as possible.
For the vegetables, they can also be roasted, or not. It always depends on what you want your end result to be. For example, you wouldn't roast vegetables for a white stock. Similarly, tomato paste is only added to brown stock because it would give white stock an undesirable color. However, tomato paste, or another acidic component is very important because acid helps dissolve connective tissue which will give your stock more body(gelatine). Beware, too much tomato paste will cloud the stock.
I have read that vegetables also give out most of their flavor after 45mins - an hour. After that, the cell walls break down, the vegetables turn to much and clouds the stock and also adds a slight bitterness. But I have never seen anyone add vegetables to a stock during the last hour of cooking...
Maybe I'm looking too much into this stock making process, but never forget that it's the base for so many culinary preparations and without quality ingredients, you will never make quality food.
A quality stock should be evaluated on four things:
I hope some of this information will be helpful to all of you. Bon courage!