The highest rank you can obtain playing only offline is C6.
The highest rank you can obtain playing only online is C1.
If you're hosting the game, you can kick out other players by going to
the options menu.
If you complete a stage offline, you can access the next stage online,
and vice versa. The only exceptions to this are the bonus stages which
you unlock by obtaining par times before the stage, and the Lorelei,
which you unlock by completing Minsk.
A triple head-shot is possible. Anything beyond that is possible, in
theory, but good luck trying to get the enemies to not kill each other.