I'll answer the questions I can...
The rental is $55.00 a day of which they refund $20 when you return the key
You can use the
scooter at either park and just drive it between them.
You can not take the scooter past the security check tents on either side of the entrance plaza.
The rental location varies depending on anticipated crowds. If it's going to be a busy day, they set up a tent to the right of the entrance gates of DL so you would rent it and then go into the park. For slower days, you enter the park first and the rental place is to the right hand side.
You can also rent inside DCA on the right side near the lockers/restrooms.
I have heard of a couple of offsite rental places (check the FAQ) and I believe they deliver it to your hotel. I don't know about loading an
ECV on the ART bus, but I would assume they are accessible. Be safe and call ahead. The offsite rentals are generally 3 wheel scooters that are much easier to drive and control. The ones at the park are 4 wheel and a little harder to maneuver, but not too bad (they are heavy duty models btw).
Using them in the park is pretty easy.
When you rent one at the park, they will give you a guide for guests with disabilities that tells you where to go to get on each attraction. Most of the DL rides you enter through the exit gate or a separate entrance and bypass the line entirely.
At DCA, all the rides are compliant and you get in the queue with everybody else.
At DCA there are curbs in the Hollywood Backlot area you need to watch for.
At DL, be careful on Main St. More than one person has driven off the curb, and I know of one lady from the boards here who actually tipped it over going off the curb and had the scooter pin her to the ground.
I would strongly suggest you take your father to the grocery store or some other store that has ecv's for customer use and let him practice for a while.
As for which is easier...it's a toss up. It's easier not to have to drag the ecv from the hotel and have it loaded, then unloaded. But if your father can't manage a short walk, he'll need it.
The ART bus drops you off fairly close to the entrance plaza. I would say he'd have to walk maybe 75 yards or so at most (to DL rental, DCA is a little further).
Wow that was long...If you have any further questions, just ask.