Hey Bruce,
I'm a complete lackwit when it comes to grading which can be determined by some of my other posts in this Forum. I say this because you ask whether or not you should be on the hook for someone that can't determine if they're satisfied with a book you sold them within the 7 day time period. If this is your only policy then you're fine - 7 days is B & W and brooks little argument.
You may or may not have read the thread where I received a Hulk 181 back from CGC that was missing the MVS. I didn't send that book into CGC until three months after the auction's close. If you had been the seller (doubtful the MVS would have been missing ) I would have had an easy return, but would it be right if I paid the grading fee? This may be a special case and therefore not a good example.
I can sympathize with your stance on you taking all the risk in CGC grading if you change your policy to cover the buyer's grading fee. I am one of those people that think the buyer is always right (except in some extreme cases) so yes, I think you, as a seller, should take all the risk.
From your current stance I would recommend that you stick with your first option and remove the return option for books that are CGC'd within six months (realize this may hurt sales -at least in the short term).
Bottom line - No easy answers. I will say that I think you have a good return policy and wouldn't hesitate to bid in one your auctions! Good for you for at least asking buyer's opinions and not arbitrarily changing your policy on your own (though perfectly in your right to do so - obviously!)