-1. You cannot set it up so that he covers your entire loss only if you lose money
First of all, I never wrote anything about covering yourself from losing the principle. Heter iska was introduced to the conversation as a way to pass on any interest charges on the cc to the final borrower, which would certainly be asur without a heter iska. In fact, I pointed out that if you can't get the guy to pay you the principle, you're not likely going to get him to pay interest charges.
Second of all, while you can't set up a guarantee of your original loan, you can make it very difficult for the 'borrower' to
halachically (very important caveat) not have to pay you the principle. This would only be necessary in the first place because of the investment structure that the heter iska creates. Without it the borrower would be 100% liable to you for the principle only in beis din (assuming you can prove that the charge was for him).