Hm... Had to look this one up... What makes sense is:
All Dragon Rulers have a similar effect of "You can only activate 1 of -insert Dragon Ruler's Name here- once per turn." Meaning that the returning effect can only be attempted to be activated (as a mandatory effect) once each turn. However, even if the dragon ruler was banished and you used its effect to search, since it is a mandatory effect, it would still activate and return (unless of course LaDD is on the field and able to negate).
LaDD fails when it comes to Lightsworn though, since it's a mandatory effect to mill each end phase.
@ "C"
Rabbit returning isn't a triggered or activated effect. It's the resolution of the effect.