1. Earth having no moon won't really effect it.
2. Yes, you can have as many moons as you'd like, but if you place them too near to each other, be warned that there's a high chance of collision!
3. US2 currently does not simulate life at all and climate simulation is limited to Earth and Mars, so no.
4. Yes, you can do this, there's even a stress test with (I think) 100 earths all in the same orbit (don't ask me how this stays stable as I have no idea)
5. Depends. If you want to change the sun type, due to the mass change, you'd have to change the orbit back, but having different types of suns won't effect Earth much other than if you change the sun type and keep the orbit as-is, the different luminosity will cause different temperatures, but thats about all.
6. You'd have to design this yourself but other than temperature it wouldn't effect Earth much.
7. You can do this easily.
8. Yes, you can do this by using the launch tool or encounters tab in create for the star and putting planets in orbits around the other star.
9. Effects like that aren't implemented yet but may be one day, depends on what the devs want to do.
10. Solar flares are in game currently but they don't effect anything.
11. This most likely could be done but you'd have to do the velocity calculations yourself beforehand to make sure it happens, as such an occurrence is extremely unlikely on its own especially if you don't have thousands of asteroids is highly eliptical orbits that cross Earth's or go very near it.
12. This can be done but it takes a few years for the effects to reach the sol system and even at that aren't much but it can be done.