An armored turret could not protect the whole submarine

Could an armored turret make a cruiser submarine safe enough to fight armed merchant ships?

No. It might protect the gun crew, but the pressure hull and the rest of the surfaced submarine would remain vulnerable to three- or four-inch fire.

Arming merchant ships with three- or four-inch guns made a surface engagement dangerous for any submarine. A protected turret could shield the gun crew and streamline the mounting, but the pressure hull and other exposed parts of the boat would remain vulnerable. Armor heavy enough to stop merchant-ship gunfire would add further weight to an already compromised design. Protecting the gun position therefore solved only one part of the survivability problem.