"Anyone see any difference in natural hog/sheep casings from Butcher & Packer or The Sausage Maker?"
I have only used natural casings from Syracuse Casing Company and the quality has been outstanding. They are a bit more expensive but IMHO the additional expense is well worth it. They come pre-tubed so will never get tangled up and are super easy to slide onto the stuffing horn. Also, they are ready to use in a matter of a few minutes, which is critical for me. Most of the time when I make sausage it's spur-of-the moment. I've read about some people soaking their casings for up to 5 days before using. That's not an option for me as I can't ever plan that far ahead. The Syracuse casings are ready to use in 10 minutes: rinse salt off and soak in warm water for a few minutes.
"3. What is exactly fibrous casings used for? I know I should know this, but...."
Fibrous casings are collagen, and exactly that: fibrous. They are typically used for things like Summer Sausage that is going to be smoked. The fibrous aspect allows smoke to penetrate the casing and get into the sausage. In this way you can actually get a true smoked flavor versus using non-fibrous casings, which you may as well bake in the oven from everything I've gleaned here. I make some sausages with collagen casings and always opt for fibrous.
Hoping to help a bit,
Robert