It seems like you are imagining some kind of perfect scenario where marth has his jump and is not that far offstage and coming from above, it seems... that's hardly edgeguarding. At that point you are above Ganon and will eat a uair regardless.
When marth is actually offstage, and has his jump, he has the ability to DJ fair (sometimes dj nair is better). This is beyond predictable. Seriously, no matter how you look at it, marths recover is SUPER linear. Like I said, he is safest when he can DJ sweetspot the ledge, and Ganon covers that pretty well.
edit: recovering low = must up-B to grab the ledge. The invincible ledgedrop uair is 100% guaranteed and easy as hell. if you don't have the time to grab the ledge, lightshield edgehog and cover options from there. Cake.