Some of your 'shortcuts' are not good ideas. Definitely start with cold water. Definitely bring up the temp slowly. Definitely do not boil. Do add aromatics upfront to the broth, but remove them as they get mushy so they don't cloud it.
Standard ratio for beef broth would be: 8 pounds of bones to 6 quarts of water to 1 pound of veggies (onion, leek, carrot) to one 'boquet garni', essentially garlic, rosemary, anise flavoring for pho, and bay leaf, plus whatever else I forgot.
If you have 'pond water', which I interpret as thin-tasting, you probably put too much water in the second time -- this is fixable by slowly evaporating out the water until it gets to a good texture. If you skimmed properly, it will be clear as you do this. I will typically strain through a kitchen towel or cheesecloth as the liquid evaporates down.
I'm guessing you put in like a gallon of water, so you had like three or four times too much water.
As a warning which you probably already know, you are not going to be able to duplicate your local pho joint's broth -- the broth recipe is the thing for pho makers, and they probably have a bunch of tricks they use, including using a neverending supply of yesterdays pho, that you won't be able to do at home. That said, you should be able to get a good beef broth if you follow some basic rules for making stock.
Rendered beef fat can be used in a lot of ways. You can use it in place of oil in a lot of recipes, but finding out which ones you like will take some experimenting. Around our house, I use rendered fat from beef or bacon in place of oil when sautéing, for example with onions and peppers, garlic or mushrooms. I've also used it to add some kick to gravies.
You could use it to pop popcorn, which I've heard is delicious but unfortunately have never tried.
I've had pasties made with beef fat, and they were delicious. Mashed potatoes as well.
As far as fat ratio goes, it's better than butter, but not as good as other fats. Beef fat has a high smoke point and is suitable for frying.
The table below is based off of 1 tablespoon. Ratio means saturated to unsaturated. Smoke point can vary depending on a lot of factors (olive oil can range from 300 when unrefined, to 375-450 when refined depending on quality) but the table below should be a good guide. For the oils, I took the refined numbers.
sat mono poly ratio smoke
Canola Oil 0.9 8.2 4.1 1:12 470°F
Olive Oil 1.8 10.0 1.2 2:11 450°F
Chicken Fat 3.8 5.7 2.6 1:2 375°F
Duck Fat 4.3 6.3 1.7 1:2 375°F
Lard (pork fat) 5.0 5.8 1.4 5:7 365°F
Beef Tallow 6.4 5.4 0.5 1:1 400°F
Butter 7.2 3.3 0.5 7:4 350°F
(fat source) (smoke point source)
A lot of people mix it into their dog's food, or use it to feed birds.
Seach for tallow if you want to find recipes that specifically use it.
Best Answer
Ideally it is simmered, but the proof is in the taste really as there's no safety issue. If it tastes good then use it, if it doesn't chuck it.