I haven't found an answer to this yet, boards or book.

For ships with turrets on differing sides, such as ventral and dorsal mounted turrets on the TY 1300, can enemy ships decide if they're "above" or "below" the ship, maintaining only one turret that can target them? Can the pilot of the YT 1300 move his/her ship so that both turrets can fire at an enemy ship? Does the "Gain the Advantage" action have anything to do with this?

I'd say it was a very creative thinking and allow it. (Think "Top Gun" upside down flying) It would be difficult to maintain for long, though.

I have a similar question (on turrets) can the pilot (of a light frighter) fire the turrets from cockpit? Perhaps with higher difficulty, and only front?

There don't seem to be any kind of "facing" rules. Even if you "gain the advantage" your opponent can still shoot at you. Over the course of the turn, the ships are still swirling around each other, giving everyone a shot who has access to a gun.

I'm guessing they put in the facing information to keep things open in the future should they decide later on to release a more detailed space combat system for those who want it.

I think facing is important in specific situations, like chase, or perhaps leaving a tunnel/hangar bay etc.

Yeah, firing arcs are largely left up to the narrative of events. In general, during a 'basic' starship encounter, ships are presumed to be maneuvering for optimal firing solutions.

This changes somewhat for Silhouette 5 ships and larger, which requires more GM adjudication for where other ships are in relation to the craft.

I'd say it was a very creative thinking and allow it. (Think "Top Gun" upside down flying) It would be difficult to maintain for long, though.

I have a similar question (on turrets) can the pilot (of a light frighter) fire the turrets from cockpit? Perhaps with higher difficulty, and only front?

Why would it be difficult to maintain? Outside of atmospheric craft like our jet fighters, there is nothing that prevents a ship from flying "upside down" in an atmosphere when you don't need to generate lift and have internal gravity so you don't have to worry about blood rushing to your head. Even more so out is space where "up" and "down" are just concepts that have no meaning without gravity to give them a reference.

Y'ever play the X-Wing games? In X-Wing Alliance particularly, you spend a lot of time in YT transports. It quickly becomes instinct to keep your enemy on your midline, so that both turrets can fire at him at once. While piloting, you couldn't really control the turrets, but they had like a simple targeting computer. You also had a single blaster in the front of the ship, and maybe you could lock the turrets forward so the pilot could use them. My memory is foggy, as it's been a long time since we've had a good Star Wars space fighting game. Ye gods, such a long time...

I'd allow the pilot to the we his action to attempt a skill check o do this.