DS14 adores lamb lung as much as my dogs do. He is cubing the lambs to freeze in small portions. DH wouldn't try it at all! It tastes good to me but the texture is similar to mushrooms. I don't like mushrooms because of the texture.
Well making head cheese out of all the head meat is a possibility. Brain not usually included in that. I think pig brains are the only ones eaten regularly by anyone anymore as they don't get prion diseases and are safe. Well poultry is safe but too small to be worth it.
Tongue is good. Mom always simmered it, then peeled it because it makes it easy. Smaller animals like sheep you'd need several for a meal. It's the best, tenderest pot roast ever.
We never bothered with lungs on anything.
Saving the caul fat is something I'd do. It's supposed to be the finest fat for pastries when rendered.
I'm a big fan of natural casings on sausages. The intestines would get used here, even though they are a hassle to clean.
Tails are a bit small on most livestock to be useful, but if I had enough they would make good soup or stock. Oxtail soup was a favorite growing up. Mom always got a lot when Great uncle butchered because no one else wanted much off the cattle he sold.
Feet when cleaned up properly can be used to make jello. I have a very old cookbook that explains how to make it. Takes forever, especially the clarifying to remove most of the meat flavor. Not worth the effort In my opinion. I'd probably just cook it down forever to make glue.