The capital tour is free. It takes off several times per hour. The guides are excellent very knowledgeable. It is the only way to see the Capital. You must be escorted and have special passes otherwise. Highly recommended but does require significant walking & of course security checks

They mentioned selfie sticks when I went through. Not sure how that may apply in your case.

I signed up ahead of time so we didn’t have to wait in line. Just showed up 10 min before our tour time. Definitely do the tour! It’s worth it to learn all of the interesting stories you wouldn’t learn by looking at the artifacts.

parking garage near the newseum and walk.. we left are car there all day and walked to all the attractions.

Ours was about an hour and a half.

Yes, the city is wheelchair friendly. You can find elevators in Metro, museums, US Capitol,..., for friends that need to use wheelchair. We attended a tour in US Capitol. One man in our group was in a wheelchair. The guide would lead him to use the elevator while the rest of use used the stairs to get up and down the building. No problem at all.

I did very well with Lyft, door to door convenience, no parking problems and distances are so short the fares are very low.

I’d arrive at least 15 min in advance

Yes, it helps. You can try to register on site.