How many grams of gelatin are in stock/broth

I'm looking for the online reference, but I remember reading in Cook's Illustrated that they were able to substitute a bit of gelatin to mimic the mouth-feel of homemade stock. I did find a beef stew recipe that used gelatin.

Based on how you described your recipe, I would say that the long cooking of chicken bones is indeed what's missing. You might get a better result if you roast your chicken breasts and vegetables before cooking them in the broth.

Duck feet will render plenty of gelatin because of the amount of cartilage, same as chicken feet. Any bird's feet are a good choice.

The method for making stock is pretty much the same no matter what you put into it, so yes, you can follow your favourite recipe for chicken stock and substitute duck feet. The only thing to keep in mind is that duck feet, like any other duck parts, are substantially more fatty than their chicken equivalents, so assuming you want a clear or mostly-clear stock, you'll have to spend a lot of time skimming.

Alternatively you can "clean" the feet and other bones by boiling them in plain water for 5-10 minutes, then dumping it all out and starting over - which is what your linked recipe suggests. I don't usually do this for brown stocks because you lose a lot of flavour, but if you're starting from raw then it won't make much difference and it's an easy/lazy way to get rid of a lot of the fat at once.