Rob,
Like you fly this HUD on a daily basis. We bought the AOA option. You are correct that is what the circle with the 5.0 is displaying. Each increment is 5 degrees with zero being at the 4:30 clock position of the circle. When the jet is in the landing mode the band appears next to and on the outside of the circle showing that you are in the correct AOA range. The triangle at the top is stick shaker and moves as you change configuration climb/descend. As you increase AOA the pointer, running from the center of the circle to the diameter, would move “up” in a counterclockwise direction. As you decrease AOA the pointer moves "downward" in a clockwise direction.
I checked my manuals looking for an answer to the vertical arrow below the horizon question and all I could come up with was a non-specific reference to a “horizon pointer”. It’s odd that none of the pictures in my HUD manual specifically identify the vertical arrow.
I have been looking at this thing in flight and noticed that it “ghosts” just like the airplane symbol and flight path vector does in a strong cross wind. I think I like your “track” explanation more than my horizon pointer answer. It’s odd that my company issued manual doesn’t address it, more likely I haven’t found the correct reference yet. If it is a track pointer then we need to correct our manual.
So much to know...........