If there are 2 or more of you travelling, I think when you factor in the cost of the shuttle, you will find that the cost of just renting a car for the entire visit will be much less than doing a one-way rental.
If you have Disney Club, use both of their discounts with National - it does work - put in both the coupon ID and contract ID from both discount offers and you will get both. You do have to take the coupon with you. We are members of Emerald Club and use the Emerald Aisle service quite often, and it works very well for us. I think my National rental cost me $22 per day for an intermediate with the discounts figured in, including fees and taxes.