Is it OK to connect the 12V Supply directly to the Actuator, and not via a relay?
That depends on the version you get. For versions B, S, and P you will need something to interrupt power to the actuator. However, I think you are referring only to version I. For that version, you can leave the power attached and the drive will not function without a signal. For either microcontroller or 555 control, you would be using the RC or PWM mode. Is the speed OK for you? The fastest is 11 mm/S, which is pretty slow compared to a standard servo.
John