Am I right in thinking that Dengar's ability on the J5K allows him to return fire in 360 degrees (as he is a turretted ship) or is it only within the forward arc?

It's just the arc:

"Once per round after defending, if the attacker is inside your firing arc, you may perform an attack against that ship."

Or are you confused about arcs? No ship has a 360 degree firing arc. Turreted ships are allowed to shoot at targets outside of their arcs.

Yep, still has an acr printed on the tile. Same with torps/missles/etc can only fire out of those printed arcs (primary) unless something tells you otherwise.

Am I right in thinking that Dengar's ability on the J5K allows him to return fire in 360 degrees (as he is a turretted ship) or is it only within the forward arc?

No. Your arc is the shaded portion printed on your base. Only within the forward arc.

360 arc is not a thing.