I'd actually imagine tipper fsmash *generally* doesn't work if you're going for that frame 1 vulnerability thing. During the ledge snap animation, that one frame of vulnerability occurs when the opponent's hand isn't actually on the ledge, but rather it occurs right before they actually grab it and are drifting to the ledge to snap it (if I'm not mistaken). If that's true, going for a tipper fsmash at the ledge during the one frame of vulnerability should almost never work if the opponent recovers low... I think? It might work against warp recoveries though since when they come out of a warp, the distance between the opponent and the ledge might be minimal, so you could still hit them during that first frame of ledge snap when they're drifting towards it. It may also work against people who come at the ledge horizontally (like fox, falco, Falcon's Side-B) if you space the tipper fsmash to hit offstage rather than at the ledge. Of course, tipper fsmashing the ledge should work vs people who have difficulty snapping it to begin with, but that's different. Is any of this logic accurate (in no way am I sure, I'm asking you guys)?
On Lylat, tipper fsmash might work if the stage is tilting down.
How "low" can dtilt and dsmash hit below the ledge, if it can?