Honestly, take the metro into town and get off at the Smithsonian stop. That places you roughly in the middle of the National Mall. They have a red bus called the DC circulator that you can ride for free and it will take you to all of the major stops. Or you can take a stroll around to see all of the monuments and museums.

My opinion is No, that's not enough time, 20 minutes at each place is like running from place to place without actually paying much attention to what is there. I suppose, depending on how you sightsee, you could run up to it look it over quick, run to the next Monument. But I like to take it all in, walk around, read the information that is posted. I can't imagine being able to read anything about each place you are seeing at the speed with which you want to go. So I guess it depends on what you want to get out of it. Do you want to just look at the pretty building and keep moving? If so, sure, you can see all these things in that time. If you want to get more out of the experience while learning about each place... .you will need to allocate more time. Enjoy your visit!

It is. But D.C. is worth spending more time in. If you also want to visit some of the museums, you should plan for some days. Enjoy your stay. It is a nice city. :)

That’s a very long drive! But yeah is possible

We found a parking garage for the day which enabled us to peaceful walk around all day with no worries

The memorial is an outdoor memorial. Thus, you may visit the Lincoln Memorial 24 hours a day 365 days a years. Rangers are on duty to answer questions from 9:30 a.m. to 10:00 p.m.

Memorial is open 24-7 but gift shop and restrooms are normal business hours I think

Walking is the best option across to the Monuments.

you can park on any streets around the memorial area, regular street parking, but read the signs, they can get quite confusing.