Fatback, salt pork and bacon are not the same. Fatback is the FRESH layer of fat running along the back of a hog. It is use sliced for barding, grind up for sausages or rendered for lard. Salt pork is a salted piece of pork, generally from the side and belly. Bacon is salted and cured.
Lard and fatback are not interchangeable. Depending on your recipe, you might be able to substitute fatback by blanching bacon to remove the smokiness and salt or blanching salt pork to remove the salt.
Not exactly. Fatback is solid unrendered fat with a distinct flavor and texture. It sometimes slightly cured and sometimes smoked. Lard is rendered uncured fatback and is basically a semi-solid oil.
Fatback is used for flavor, not so much for the oil/fats. I would use a small amount of bacon grease if fatback isn't available. Probably 1/10th - 1/20th the amount of bacon grease to the amount of fatback called for. Or if you have slab bacon you could use some of the fat from that, probably about 1/3 to amount as you would fatback, because it is heavier smoked.