Think he might get sick of being the cash cow after a while?
Just saying that most people end up having issues when “sharing” something like a car. Especially if everything isn’t going to be 50/50. He’ll probably see it as him owning the car and you paying rent on it or paying for consumables.
For cone damage, tape up the front bumper a bit and the areas in front of and in the back of the wheel wells. Unless you decide to go Takumi and drift sideways into cones, you’ll be fine.
You won’t break the S by autocrossing it. Learn how ot launch it correctly or don’t launch it hard (doesn’t really matter in an autox), learn to shift it well and you’ll be fine.