The ledge can be safely stolen in between the various Uairs can hit you even while you're still on stage (character and MK position dependent obviously)? There is also the small fact that MK could simply tornado his way back up to the ledge, while hitting your character once the invincibility frames are worn off, make it to the other ledge, all while gaining a larger lead through the Nado and wasting more time. Obviously, extreme theory crafting going on here, think of it more of an example of how little this would hamper MK if this did happen than "this will happen if they grab the ledge".
I say it isn't beatable for two reasons. Firstly, the current frame data in combination of MK's truck load of options from the ledge highly, highly suggests that he can't be beaten on the ledge. This isn't something silly like "you can beat Ike by shield camping", this actually has a second point to back it up: people are so afraid of it that they are trying to weaken it with things like LGLs. At this point, I'm willing to say it, along with MK's options, render MK unbeatable on the ledge until I get some data suggesting otherwise. Which of course, I won't get with everyone and their mother using LGLs against the few people who would be willing to try this. *shrugs*
It doesn't ignore your point, it suggest that there wasn't much of a point in the first place. As for the last bit: depends on the stage list. If you have a very liberal stage list, I believe enough stages disrupt MK's normal aircamping game enough that there could be a neutral MU against him. Not on a conservative stage list though. And seeing how few stage lists I feel are liberal enough to have a shot at this, it might as well be none. It's basically Nova Scotia that has a shot at it. :\
Basically: yes at the extremes of what the smash community will do. No with what typically happens, especially around the MK heavy areas like NE US.