I'm not sure my recommendation will be popular but IMO, you shouldn't agree to the mutual and also, IMO, it's a bit nervy for a seller who has clearly mis-described an item to request a mutual.
Had the item be accurately described, you either would have known what to expect or you would have hit the back button and found another seller with a bag in better condition. You wouldn't have had to go through the hassle of disputing and returning.
Although you don't want the SNAD item, it's time for Linda and her staff to get the message that their listings are inaccurate and more attention to detail needs to be paid.
Perhaps if some of the FVFs of returned items hit her in the pocketbook (no pun intended), she will put more effort into accurately describing her items, verifying authenticity and responding more promptly to emails.
By requesting mutual cancellations from her buyers, she has no incentive to change the way she does business, i.e., just offer "no questions asked" refunds, get the item back, have a happy buyer who is glad to get the refund without a fight and continue on her merry way.
She is only out the listing fees, which are minimal in comparison to FVFs.