The quantity issue is because your product is priced at 0.00. To prevent abuse, our cart will only allow one of any zero-priced product to be added to a cart. If you change the item price to something greater than 0.00 in Seller Admin, the &quantity= parameter will work.
As for forcing the cart cookie to reset, or deliberately causing the cart to "forget" its contents between views, I have assigned the matter to our Lead Developer's attention, as that would be up to him to consider and offer a solution, if indeed one is possible at all. You might be able to figure something out on your own by examining the cookies our cart sets in your browser. Since our standard browser overlay works "inside" your page, cart-related cookies would be set as a first-party cookie with your site's own domain. I think ej_ejc_cart_idXXXXX is the main cart-tracker cookie (where XXXXX is your client ID, apparently 48651 in your case, looking at your sample URL above), and it appears that deleting that cookie in the buyer's browser should clear their cart contents.