sharing stuff

If I (for instance) create a map with dynamic lighting set up to show another user what they can do with Roll20, could I then give them the map to use in their Roll20 (assuming they had the same level of use as I do)?

The graphics you upload are in your user library, not theirs. The only way to really do this is to set it up as a Marketplace item for sale. You could put it in a game and make them the GM of that game, but it's not the same as being in a game they created. Roll20 has some pretty strong barriers to casual sharing of art and creative endeavor. This is by design, and allows them to pursue licensing contracts and to protect creators.

Is there any way that I could then set up the demonstration in their game?

I believe you could do that if they created the game, had the necessary permissions and then made you a GM. They'd need to place the map down, but after that you, you should be able to show them how to set up dynamic lighting, etc.