Just browsing real quick, didn't read every comment.
I think Spears should act in a standard way when front facing. Semi-functional at Side attacks, and then "Goofy" as the Dev put it from back charges. There is zero reason Cavalry should be able to front charge a Spear wall/Phalanx position. Perhaps if you want to gamble high level Cataphracts vs peasant levy spears and gamble Armor Vs Crap weapons.
The AI does some what understand Cavalry charge from behind, I've seen it. So why they would make the spears useless is poor judgement. Renders the units useless, and they are specialized units to begin with.
Make Spears Brace from the front 100%, of the time from the front. 25-50% from the sides, and 0% from the back. Making a Spear wall, or circle should be a viable tactic that requires either Archers, Skirmishers, or Infantry to break. You shouldn't be able to just run on in with Cavalry.