^This^ is partly what I learned this morning while listening to the recorded greeting of an outside golf cart rental company. And you've confirmed in a much more straight-forward way.
I left a message as instructed, but haven't heard back. The recording said they meet customers in a 'safe location' (or did I make that up?) ten minutes away, where customers hitch a trailer to their vehicle. I'm assuming a hitch is needed. My aunt is driving to WDW so she'll have a car (I won't)...I'm not sure if she has a hitch...but we'll only be there for two days...here's hoping those internal buses are as much fun as they were last time I was at the Fort. When I was 7.
Does anyone have an educated guess why the parking lot can't be used? Perhaps an insurance/legal policy? I'm totally fine with the whole situation, but I'm curious, curious, curious!
Depending on the vendor, if your vehicle does not have a hitch to take their trailer with the cart on it, you can take their truck/trailer combo (leaving yours at the meet point), drive to your site, drop off the cart, drive truck/trailer combo back to meet point, and get your vehicle. That's a lot of work on you for an off-site vendor rental - although sometimes the vendor has a cart being used by someone else who is leaving and the vendor can tell you where to find the cart and the keys at the Fort. It all just depends.
Honestly, for two days rental that's a lot of work unless you can't get a reservation for a Disney cart. In my head (and I rent carts only rarely), the over/under is 4 days on Disney vs. off-site.
Vendors USED TO BE ABLE to use the FW parking lot as the meet point years ago but Disney put a stop to it because I think word was getting around the camper community about the savings the off-site vendors passed on to the customers. First time I ever rented off-site we met at the Fort lot. So IMO for purely anti-competitive reasons, Disney forced the vendors to meet guests off-property to make it more inconvenient for their guests. Because they could. Which then makes Disney's rentals more convenient and worth a higher expense.
A few folks have had bad rentals with off-site or poor support. Of course with Disney it's usually top-notch service and support.
Honestly, the choice is yours. If you can't get a Disney cart reservation and the vendor can get you one for a two-day rental, it's up to you. If both were available, I'd pay Disney for that convenience factor.
Good luck!
Bama Ed
PS - I've also been called and told by the vendor where a cart is and how to get the keys which is better than drive-meet-pull trailer to FW-off load cart-pull trailer to meet-drive back to FW. But it's all up to you.