I think the Defiant could have been turned into a good night fighter. But it would have needed some forward-firing armament. It could have been developed with the four 20 mm cannons and could have had the turret replaced with a radar unit. It would probably have out-performed the Bf 110, which was a good night fighter.
You could have had two single-seat Defiants for every Mosquito you were willing to give up.
Personally I think it was a large mistake for Boulton-Paul to have not designed in a forward-firing gun provision, even if that meant adding more wing area; I would never have funded the prototype without some forward-firing guns, but that's just my opinion. Also, I'd have wanted more Mosquitos, not fewer, so the Defiant would never have gotten off the ground if I were making the decisions. Still, it could have been made useful. The question is would it have been more useful than what really happened? That's a what if I can't answer.