Are there NPCs in deep space

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.

Whilst I have not done this myself, I have read several times on the official forums that other players have. If you have the items the ships are seeking, they are magically taken from you and you are automatically paid for them.

I can only assume that you get paid a considerable amount more than you would get selling them at any station or you could end up quite upset if you lost out on potential profit.

This wouldn't stop situations where you had bought the cargo for a "Please bring me X tons of Y" type mission from a bulletin board and the cargo was taken from you.

Edit: Confirmation of the "fly close" mechanic. https://forums.frontier.co.uk/showthread.php?t=96996&p=1507483&viewfull=1#post1507483 This thread also clarifies what is a "luxury item", in this case players are trading Progenitor Cells with these mobile markets.