Run off fair, with a fastfall as appropriate, is supposed to catch DI that keeps them close to the stage, but offstage, and attempts to stall or sweetspot. Watch the DI if you're thinking about it.
You shouldn't be thinking of these things as being "guaranteed" at a certain percentages considering how much changes simply from how close you are to the ledge, for instance. The further from the ledge, the longer it takes for you to run off, but if you're right on the edge, you're far less likely to hit them with any of this by running off.
The answer is literally you can run off fair if you can get the fair out below the ledge far enough before they could go past it with an air dodge and not die anyway, which can be pretty early. You go for run off fair (with a fastfall, potentially) to catch a jump and also cover shine stalling, since you can catch both to the point shinestall doesn't let them DJ to the ledge.
Run off fair is beaten by either DIing further away, DIing to not go offstage/fall off and grab ledge, and earlier/when Marth is further on stage from the throw, jumping and airdodging or attacking before he can get the attack out.
Double jump dair is beaten by not jumping into the dair, or by Marth being too far on stage to properly run off in time.
It's worth it if you think it will hit since they should die if you hit them with it.
You should usually fast fall the fair since you're literally covering more vertical space, unless you think you were going to miss anyway, in which case you shouldn't have run off stage doing an aerial. With that said, fast fall *after* inputting the fair. If they DI further out, you could delay the fair and not fast fall I guess, but it almost certainly would have been better NOT to go offstage (or to run off and double jump back onto stage) if you couldn't do the aerial immediately after running off.
These edgeguards aren't just effective vs these characters btw, you can do them in Marth dittos or vs Sheik, for instance, and it's potentially less risky in those MUs.
Ultra simple version: Does it look like they can double jump to the ledge or over it? If so, you can go for either the fair or the dair, doesn't mean they'll connect, but you can. If they explicitly respond incorrectly, the dair works, if they don't respond correctly, the fair works.
When you catch someone's DJ with either of these at lower percents, it's probably because they missed the timing on their jump one way or the other, at least in part.
