The rules say that Lorn is an imperial officer controlled by the rebels. Does this mean that they get the deployment card and can perform attacks as the one action at the start of the status phase?

Lorn is a rebel ally, using the Imperial Officer deployment card (not listed in initial groups). Instead of activating like a normal ally, at the start of each status phase Lorn gets a 1-action activation. The action can be any action available to him - Move, Attack, or Order.

Thanks, and this wouldn't prevent the rebels from bringing another ally?

None of the allies provided by the missions count as the one ally the rebel players can choose to take into each mission (if they have any).

Allies provided by missions also do not give the rebels threat.

Special Setup tells if there are any ally restrictions for a mission.