Elite dangerous powerplay universe, are there single player universes

I can only definitively answer part (1), and that answer is:

Yes, once you have explored a system you retain that information and it shows on the System Map for you.

I understand from the game forums that the first players to survey a system get more income from doing so than later players - but that later players do get some (presumably this data is confirmation of the earlier surveyors' findings).

I recall from memories of statements by the developers (can't cite, sorry) that in the released game, systems that have been explored and see a lot of player traffic will eventually have space stations built and come under faction control. I don't know whether the faction that players sell the data to has influence on the faction that moves into the system.

There's a mechanic in Elite: Dangerous called "flight assist". Enabled by default, this causes a ship to behave as if it was in atmosphere. However, you can disable flight assist and your ship will behave as if it was indeed in space.

In fact, disabling flight assist is a common tactic, boosting ahead to before disabling flight assist and turning around, allowing a pilot to fire at a chasing ship while still maintaining distance.

So the real answer is that Elite: Dangerous uses a bit of both.