Are you intending for this to only happen once per game? Because as he reads, that would activate every battle phase he entered, even if he had already been shuffled. To get the ability how you'd want it (if I assume correctly), it would be "Once per game, if Jonah is being discarded, you may shuffle him into deck instead." I think at least...
Why not give him something though that more fits his story, like:
"Once per game, set Jonah aside. On return, convert a number of evil characters to a brigade of your choice, where the number is equal to the number of turns set aside. Cannot be interrupted."
This would cause you to 'waste' a rescue (and is not optional), much how Jonah wasted time rescuing Nineveh, since you could only do this by pushing him into battle and you don't have a choice. However, being able to have him come back and convert people in the end fits the story, and the ability lets you make the most of that 'waste'. The once per game means you can't just do this forever. Also, by not saying "up to" you can't stop the power of conversion from hurting you if you wait too long.
