The slide stunt doesn't involve turning. The description actually states that the ship does not change its facing. If you look at the diagram, it shows that the ship stays facing the same direction, but moves sideways along one of paths to the side, either forward-port or forward-starboard.
Essentially it's a strafing move, but as you're (presumably) already moving forwards, you actually keep moving forwards as you move sideways. Each stunt appears to use the total piloting action for that turn, so I don't see how you could move port then starboard in a single turn.
From the diagrams and descriptions, I think it's clear that the only stunts that allow turning are Flip and Burn, Flyby, Evade and Turn in Place.
The starship moves up to its speed in the direction of either the forward-port or forward-starboard edge without changing its facing. To perform this stunt, you must succeed at a Piloting check (DC = 10 + 1.5 × your ship’s tier). If you fail this check, the ship moves forward up to half its speed and can’t make any turns.
Emphasis added. Note DC is adjusted for the new FAQ.
No, a starship may not slide port and starboard in the same turn.
No, a starship may not turn (change its facing) if it does a Slide stunt.
It is required for the full movement because that is the nature of the stunt. Do note that a starship does not need to move its full speed. It can only move 1 hex.
If you want to circle strafe with a starship that has worse than Good maneuverability you need to use the Maneuver stunt to reduce the turn rate.