If Tryndamere is brought in as a defender via Ride the Wind, wins combat, conquers the battlefield, and deals 5+ excess damage, does his ability trigger to score an additional point?

No. Tryndamere's ability reads 'When I conquer after an attack' — this triggers only when the Attacker conquers. Since Tryndamere conquered on defense (not as the Attacker), the ability does not trigger. Only 1 point is scored from the normal Conquer.