A random castmember can be a wildly inaccurate source of information. At the Fort, I have come across folks working in the check in lanes, or in the trading posts, who know almost nothing about the Fort (which is pretty amazing, and sad). I have rented carts from the Fort four times since October, 2008, the most recent being last month, and I assure you that there are no longer any golf carts for rent at the Bike Barn. Rentals are handled at the time of check in, if you are picking up your cart simultaneously with your check in, or at the Concierge window at the Reception Outpost, if you are beginning your rental on a day other than your check-in day. On the occasion of my last rental, the young lady who gave me the safety spiel for the cart confided that she had been working there for two weeks. The poor thing did not know the most basic information about the place where she worked. Unfortunately, WDW ain't what it used to be.
See above regarding pick up. As I said earlier, there used to be a night drop at the Bike Barn, so I see no reason why they would not allow you to return the cart early to the Reception Outpost. I also see no reason for a night drop box anymore, as the Reception Outpost is staffed 24 hours per day. I am sure that this question can be easily answered when you pick up your cart.
Thanks for the vote of confidence, Denise.
TCD