What is the issue? It seems like you have everything you need already typed up; you just have to work out the logic in more detail and code it up.
For instance, ‘Turrets will use the prop_manipulator_device to detect when props or structures are damaged and respond by targeting the attacking agent.
Turrets need to dynamically change targets based on the current threat level (players attacking team members take precedence over guards).’
If this is the logic you want to use, you can use the damageable interface of the prop to subscribe your own custom OnDamaged(Agent : agent) funciton to its DamagedEvent. Then in your OnDamaged function, you can check if each new attacking player is a player or a guard, switching targets depending on that logic, and you can even add them to an array of agents to keep track of the targets each turret should cycle through after its current target is eliminated.
Also, just curious, is there a use for the == in verse, or is that perhaps a bit of c++ coming through here? 