Welcome aboard! Lots of info there... One thing that stands out to me is the brain shot.

You really need to see a cut-away of a buffalo skull to understand how low the brain sits. Depends on the angle of his head, etc... but the brain sits low in the skull (much closer to the neck vertebrae than it does the boss).

For example, with his nose up high, you would have to go through the nose to get to the brain. Head down (which is usually the last few feet) through the boss would work. I'll try to find some pics for you.

Although they don't show the brain location, here are some good photos (and comments) on shot placement at a variety of angles.

http://www.safaribwana.com/MEMBERS/buffalo%20shot%20placement.htm