There are benches and seating around the park. I didn’t explicitly see wheelchairs/scooters. The waterfall wouldn’t be accessible for those on wheels, but I think most of the rest of the park is.

We had no issues with wifi

Good question. There are supposedly buses that leave from Santa Barbera, near Alaquela, fairly early & that come back in the late afternoon. However, it's very difficult to get real information about the schedule ???. The best thing seems to be to have a rental car OR to try to take the bus but be prepared to hitch hike in case no bus arrives! People are friendly & helpful for the most part, so the chances that things will work out are pretty high!

Yes, although suitcases are always at risk if you leave them in a car, La Paz has a private lot and a parking attendant that was there the whole time we were. We did the same thing, stopping here from San Jose to La Fortuna. We always carry a piece of black fabric with us, and cover our luggage with it. When you look in the window, it makes it hard to see that their is anything in the car. We take this fabric with us on all trips. Just a cheap piece of black fabric from your local fabric store works great.

Yes, I do believe it's free for all hire guest.