I don’t know why people go out of their way to make things complicated, especially the experts. Scum? What stinking scum? Why is scum important? Sorry to be frustrated but it isn’t this hard. I guess the experts have to justify how smart they are, at least in their own minds.
When you boil about anything a foam will often form on top. That’s how I can tell jelly is getting ready to boil, a foam starts to form on top. Same thing is true on practically anything I boil. That’s just the air bubbles coming up and picking up something in the way up to form a bubble. People will say that’s the impurities. There is nothing impure about it, it’s just stuff in there and perfectly safe to eat. I don’t get scum when I make broth but I may get foam on top.
I don’t know how tender that meat is. Simmering two hours it should be cooked but it might be a bit tough, I don’t know. Try it and see.
With the veggies, bones, and skin simmering together you are making broth. Let it simmer, covered so the liquid does not evaporate, for several hours. How long? I don’t know and I don’t think it matters that much. The longer it cooks without the liquid evaporating the better. More of the flavor will cook out of the bones and other stuff. I like a minimum of eight hours, others may go a lot less. I practically always cook mine overnight in a crock pot, 12 to 14 hours. The batch I just finished canning before I sat down here cooked for about 20 hours. I started it yesterday a little after lunch.
After I cook it I strain it through a wire mesh colander or strainer to get the big chunks out. Then I put it through a fat separator, something like this.
https://www.amazon.com/OXO-Good-Grips-4-Cup-Separator/dp/B0002YTGIQ
Then I strain it through a few layers of cheesecloth to get more of the bits out. You don’t have to strain it through cheese cloth, some people like those bits in the broth. You can always get into an argument if it is a broth or stock if you leave the bits in. I don’t care what you call it, I take those bits out. Some people leave them in.
That’s it. You can now use it, freeze it, or can it for later.