1. Gunners can utilise more than one ammunition type. The utility of the Gunner as a frequent question, I think a single Gunner has their place on a crew. Some people won't use a Gunner at all on their crew and some Captains will use two Gunners with some ship builds. As a new player, it's best to start games as an Engineer and talk to your Captain before deviating too far from the vanilla player role (spanner, mallet, extinguisher, heavy clip, spyglass).
2. Play the game or read the website, see link at top of forum.
3. It's a hot weekend in Europe, many players also have offline lives. The population you see will be very dependent on when you play. The population you see "Waiting" does no count those already in an active match.
4. It will be payable DLC for those that have not already paid for it as part of the Kickstarter project.
Question 2 is quite silly.