I agree with this entire post except for Marth having a viable option to counter sharking... I just don't think dair is very practical in this match-up to counter sharking. Maybe it is practical. I'll take your word on it I guess. I agree with the stage control bit, but MK resets the situation when he gets to the ledge. I agree that Marth has one of the best ledge traps in the game, but MK is... MK. He can just... go to the other ledge! Or abuse his jumps, etc. Although I can understand that although MK has these options to return from the ledge to the stage, Marth still does indeed gains control of the situation, enough to take advantage of it.
Another debate in this match-up is MK's edgeguarding on Marth. Can't MK drop down nair when Marth up Bs? Also, Orion told me this: When Marth recovers with up b, you can drop off the ledge and tornado. It either puts Marth in a position to get daired or up B stage spiked, or it forces Marth to recover above the ledge iirc, where he has his up b landing lag when he lands, which can be used to get him back offstage. It was some crazy theory-crafting that he came up with, but it seemed pretty practical because he has pulled it off on Minty many times.