Normally, you wouldn't add sorbate and campden to the vessel the cider is in, as there would still be lees (sediment) and tons of yeast in that.
You'd take 1/2 teaspoon per gallon of sorbate and 1 crushed campden tablet per gallon, and dissolve that in a little boil water. Once that is dissolved, put that in the keg and rack the cider into it with the tubing to the bottom so it fills from the bottom and swirls gently to mix (without splashing). Then, after a couple of days at room temperature, add your sweetener to taste.
Then it should be all set.
You probably don't have to stabilize with sorbate and campden if keeping the keg ice cold, but some wine yeast are active in the 40s so to be safe, stabilizing is a good way to do it. I don't like the taste of sorbate myself, but I don't drink sweetened ciders or wines either so just keg them dry.