Can your fleet's admiral and a named character be on the same vessel? Could Garm Bel Iblis and Adar Tallon be aboard the Paragon for example? Or does Garm fulfill the role as the Paragon's commander as well as fleet commander?

Any Ship can have your Admiral... Which is why there is not an "Admiral" upgrade slot listed on any Ship cards.

Ships themselves do not need their own individual officers - its just that they *may* if their Upgrades allow it.

You can have Garm, Adar, and indeed then, anyone else who fills the other positions - so long as you only have 1 of each kind per slot allowed.

Yes thats right. Your commander does not take up a personnel slot on the ship. You can still have personnel on the ship even if the commander is on the same ship. However you can only have 1 commander total in your army, and what ever ship he joined becomes your "flag ship".