Hi,
1. Coupons don't apply to products but to the whole cart.
When you restrict a coupon to a product and turn on the "Coupon percentage applies to product only" setting of the coupon, the amount of the coupon will be based on the percentage entered in the coupon and the sum of the products of the cart listed in the products of the coupon. But you could also enter a fixed amount in the coupon. That way, regardless of the quantity purchased, the amount of the coupon won't changed.
2. The coupon system on the checkout doesn't support multiple coupons. However, the extension AWOCoupon, which integrates with HikaShop allows for that with their coupon system. So I would recommend to have a look to their extension if you need that with HikaShop:
awodev.com/products/joomla/awocoupon