You would need to ask, I am not sure.

I believe they don't have either. Maybe call for information, in case they provide other means to accommodate.

I didn’t see any when I was there. Try calling and see if they offfwr it. The staff we encountered was very nice and helpful!

Use Uber or Lift. There are no nearby metro stops and I have no idea which bus lines may stop near the Arboretum. Further, the area surrounding it is dicey to downright dangerous so I would not advise waiting outside the gates for some bus.

No, but in warm months there is a snack truck.

University of Maryland in college park is about 20 minutes by car in good traffic.

Free parking everywhere. There are many parking lots near each collection, but you can park on the side of the road almost everywhere. Signs will tell you if you can't park somewhere, but basically you can pull over (not too far off the road) anywhere and just get out of your car. Kind of amazing, actually.

The admission price is $0. Plus free parking everywhere (with some limits with regard to locations).

I have been to the "Arb" a bazillion times, mostly to walk my dogs. There is no skywalk. It's an arboretum, trees grow in the ground, we have to walk on the ground. As one commenter remarked, there is a sort of skywalk at the National Botanical Gardens. Which is quite delightful, my teenage niece and nephews insisted on a visit 10 years after their first visit some years ago. The National Arboretum is beyond the Washington DC tourist circle, but wonderful. The National Botanical Gardens is a quarter mile from the Capitol and also delightful.

Walking the whole arboretum would be a major hike. There is public trans to near the entrance, and there is a good motorized tour through the whole thing (which is large). There is a bike share at the entrance to the arb. I walk my dogs there often, and there seem to be many bikes at the entrance most of the time. For a pedestrian arriving at the entrance, I would advise getting a bike, there are many interesting areas, separated by what would be significant walks between. If you want a few hours of walking, it's easy to see several quite distinct areas.