Yes, the backing up of these things presents two problems - #1, it's infeasible, as you stated. Secondly it would, believe it or not, be far more complicated to code than forward motion. Getting them to automatically avoid obstacles might be a good idea - we'll think about that. But overriding the player's ride with an automatic script might be annoying to some people, and I imagine they'd quickly learn from their mistakes.
I agree that speed of a chariot should be diminished if a horse dies. For the game's sake, I'll be automatically reorienting the remaining horses to the center in this event. As far as controlling the speed once one of them dies, I know of two possible ways that this could be done, and I have yet to figure out which of the two would work better. Either way, speed will drop if you lose a horse.
Chariot horses are not trained to be ridden, so I imagine that they would not be able to be ridden unless the player has some insane amount of riding skill. In cases where a cart is pulled by another sort of animal, such as an ox, which I have seen ridden, I imagine this could be much more feasible.
On charging with chariots, this is a common misconception. Chariots are not designed for charging, the horses aren't trained to charge, and the last thing a chariot driver wants to do is charge - these things were always used as archery platforms (and the archer was not the driver) and kept at a distance, if possible, from any enemies. If footmen ever did get up close, there was a guy called a runner that ran alongside the chariot and fended people off. Chariot use in this mod will be closely akin to that.
Also, in order to drive a chariot, you'll need at least one free hand, so you might be able to throw from the ride, but if you pull out a bow or a two-hander you won't be able to control the horses (it's not like you're sitting on them and can give them orders with your feet at this point). I think this would probably be OK with a shield, though, since some shields don't require a firm grip of the hand. I could make it based on the shield, perhaps, so that some types of shields would allow this and some others would not.
As far as the control scheme, I currently think there should be four controls - one to move forward/increase speed, two to turn left or right (only at speed), and possibly a way to back up. Elephants will certainly have the ability to back up, but I'm limiting it to a straight line. I'm pretty sure that with the topographic map I can keep NPCs from getting themselves into sticky situations, but the player might not be so lucky. I don't know how mean I should be to him, haha. Maneuverability, I think, should be, on average, slightly worse than a charger. Absolute maximum speed should be somewhere between a warhorse and a hunter, and that's with maxed out horses. That's just me working out balance and feasibility in my head - let me know your thoughts. Other than that, thanks for the input so far, and keep it coming