The first post on this board describes how to set up international shipping so they have information about setting up for the gsp there.
2. Update all live listings to state "International customers must pay return shipping, under all circumstances without any exceptions."
You have to be careful of what customer service tells you. That particular statement wouldn't be enforceable if the buyer opened a not as described case and the reason was that you made a mistake you would be liable for return shipping. If the case was opened because the item was damaged in shipping or the item was switched at the gsp center, then the gsp would have to refund the buyer but they don't require a return.
For remorse returns... I'm not totally clear on how this works because an international buyer can't see any return policy on the actual gsp listing so they wouldn't see the 'free returns' unless they changed their location to the US. The gsp does state that they don't support buyer remorse returns but they do say:
"The returns policy specified by your Seller in his/her listing description of a GSP Item will apply to your purchase of the GSP Item. For GSP Items located in the United States that you purchase under GSP and that you return to your Seller, you will not be eligible for, and you will not receive, a refund of any Program Fees that you paid to Pitney Bowes and Pitney Bowes will not be responsible for shipping the GSP Item back to your Seller."
That means that if a buyer did a remorse return, they would not receive the international shipping portion or any fees paid to PB so I doubt that there are very many remorse returns. If they did actually file one though I don't know how that would work and if eBay would simply go by your stated return policy or if they would make an exception for international shipping. since I don't see a place to specify free returns for domestic and not free for international. You can put whatever you want in your description but that doesn't mean that eBay will honeor it.
I realize that my information isn't what you asked for and it probably doesn't help you much but just wanted to mention that there is often more information available than the phone cs gives you and that they are not always aware of how things really work.
If I were in your situation I might ask on the weekly board hour on Wednesdays on how to handle a situation like this...if free return shipping would be expected by eBay. Something else that I just noticed...when I look at a search page as a Canadian and check off free return shipping on the left, I do see free returns on the search page itself to the left of each listing although I don't see it within the listing itself.
