You are correct. Your turret arc determines the direction your turret fires. Your special weapon (e.g. Ion Cannon, Concussion Missiles, etc) uses the forward arc; this is entirely independent of the direction the turret faces. As you noted, there are rare exceptions like the TIE/sf Fighter which must use its turret arc for its missiles, but only because its ship ability says so.
That must be a bug in Fly Casual. I've never used it, but there's no reason an RZ-2 A-Wing couldn't fire its missiles forward while its turret faces the back. In fact that's useful for the A-Wing: its turret can cover its back while its missiles cover its front.
It's also worth noting that a ship like the Jumpmaster loves to have a cannon equipped so it can put its turret on the side or back for a wide area of coverage. Likewise ships with a double arc and either a primary front weapon (like the StarFortress) or an equippable cannon/missile/etc (like the Modified YT-1300) can set their turret to the sides and enjoy 270 decrees of arc coverage when needed (though something like a YT-1300 probably doesn't want to equip a missile or rely on it for arc coverage).
I hope that helps!