Can this ship be used effectively in combat by a solo pilot?

Yes - the Defender is completely functional with a crew of one. There is no reason you must have a second crewman for a Defender any more than you would for the currently flyable Super Hornet, with the pilot tasked in managing all ship functions. The main edge granted by a second pilot in the Defender would be having someone to manage ship systems optimally during combat, whether handling the weapons and/or shields while the pilot focuses on maneuvering, or jumping into the back and conducting repairs or damage control on the fly to keep this long-ranged fighter in play over a long haul.