Coupon discount for only one product

In our commercial edition, you get additional options when creating a coupon so that you can for example apply the coupon to the order only if a specific product is in the cart.

A coupon is applied to all the cart. If you create a coupon of $2 for product X, if that product is in the cart, $2 will be substracted to the total, regardless of the quantity of that product or other products in the cart.
If you want to apply automatically that discount to the product price, then you should create a discount and not a coupon. Discounts are applied to the products prices directly and not to the whole cart. So if you create a discount of $2 for a product, then you will have a discount of $4 if you have that product twice in the cart.

Your second point can also be done with discounts. I think that you want to use discounts and not coupons.