I'm not sure if this is covered explicitly anywhere - I know for all of the geometry problems I run into I sketch it on paper then solve it traditionally.
That said, the difference between two headings is just (scan1 - scan2). You might want to use normalisation - either relative or absolute - to put it in the range you want.
On this wiki we generally refer to relative velocity in terms of its components - Lateral_Velocity and Advancing_Velocity. The pages for those explain how to calculate them, and yes, they are used in a lot of the more advanced algorithms =)
Relative normalisation is used for determining how far something is to the left or right - for instance, whether you should turn your gun left or right. It puts the angle between -Pi and +Pi (or -180 and +180). Absolute normalisation is usually used to figure out where something is 'relative to North' and gives a value between 0 and 2*Pi (or 0 and 360).
As to your last question, I'm not sure exactly what you're asking. Perhaps you want the absolute value of the heading difference? If so, take the relative normal angle first so you don't get the situation where one is on 355 degrees and the other 5 degrees, and the difference between them is 350 degrees instead of 10.