I'll start with a link to our help page for Discounts, in case you haven't seen it yet:
We will be enhancing discount settings in the future, so some of your suggestions are already "on the drawing board". :^)
Bear in mind that product-specific discounts will always take flat amount off the PER-UNIT PRICE of that product -- e.g., if the buyer orders 3 of that product and applies the discount, the discount amount would be applied to the base price before we multiply that resulting discounted price by their chosen quantity of 3. If you want a discount that takes a flat amount off the order total just once, you would need to set that up as a discount for All Cart Items.
Entering the same discount code in the same cart multiple times will not keep discounting that order. Any given cart order can have, at most, one cart-wide discount plus one product-specific discount for each item in the cart. If you configure several discounts with the same Discount Code, entering that code in the cart would apply all discounts matching that Code.
When you add/edit a product in Seller Admin, you can disable the "Let buyer edit quantity" setting, so buyers would be able to order only one unit of that item at a time. Unfortunately, there is no practical and effective way to limit the number of times a specific person can make a purchase from you or use a "generic" unlimited-use Discount Code. If we tried to block their email, they could just use a different email; if we blocked their IP, they could just reset their connection or move to a different wifi hotspot to get a different IP; if we set a browser cookie, they could just clear their cookies.
We are, however, working on other, clever ways to detect and block clearly fraudulent purchase activity without unduly affecting legitimate buyers nor requiring any special configuration on your part; when it's ready, it will "just work" quietly in the background.