I bought 1/2 pound of mussels and cooked them in a cast iron skillet with a little white wine and some shrimp stock on boiling heat.
Almost all of them opened after about 15 minutes, some did not. The ones that opened were done perfectly, and those were the ones I served.
Still, in the future - should I just throw away the rest, or would a few more minutes in the skillet be in order?
You should throw them out. Mussels that don't open were quite possibly dead and decomposing for an unknown period of time prior to cooking. You don't want to take a chance here.
Just in case future readers don't know, mussels, clams, and other in-shell shellfish are still alive when you buy them, and they should be still alive when you cook them.
In the future, I would recommend the following: When washing the mussels before cooking, if any are slightly open try gently closing them. If they react by snapping closed themselves or they stay shut, it means the mussels are alive and well. If instead they react by immediately popping back open, throw them out (they are dead). I've found that the majority of the dead mussels are slightly open before cooking; they rarely are completely, tightly closed. Using this method, when you actually cook the mussels, you have a reasonably good idea that they are all alive and therefore safe to eat, regardless of whether or not they open all the way during cooking. After cooking there will likely be none that are closed, but if there are, there will likely be few and you can discard them if you like.
When mussels don't open when cooked, it means that they were dead before you cooked them. Don't eat them you risk becoming very very sick.
A friend of mine (professional cook) told met to steam mussels extra hot, lid closed. In about 5' they should be open and done. They are done when they open. Don't leave them cooking any longer than necessary as they'll toughen up.
As far as I know, if you spray fresh live mussels with cold water they will open slightly. If they don't open, throw them out.
