FORGET ALL THE OLD STUFF I SAID. I THINK I FINALLY GOT IT.
Alright, after labbing with boss and other md/va players, I've reworked my ledge game to something a little bit more consistent and likely closer to what you've all been already doing. I'm MUUUCH more confident in my findings this time.
Roll with your back to the ledge and hold shield around when the opponent grabs the ledge. Only use fludd and fireballs to deter high or quick recoveries until you get there.
Typically, this telegraphs an attempt at an easy grabbing of the standard get-up for a backthrow. So most players will use another option.
However, with your back to the ledge you still cover all the options.
If they ledge jump, you can REACT with a uair. They have to buffer an aerial or airdodge to avoid it (which can also be punished), and this uair true combos into Upb beautifully. Killing while true comboing as high as 150% in training mode. (It is however training mode so take it with a grain of salt)
I'm not exactly sure why, I think it's because the uair tends to hit with the back hitbox here thus better angle, less endlag, etc.
Furthermore, ledge dthrow has EXACTLY the same knockback as dtilt, meaning true combos into uair > dair to kill around 60-100% as well as better strings and damage overall make getting a grab here even before bthrow kills a huge benefit. (Not to mention it's also a di mixup since people tend to DI in expecting bthrow, making for even better combos)
*Note, even something as minute as a character model grazing you can push you off the ledge and take away this godlike dthrow set-up.
Now, for rolls you can usmash, or grab often. But the most consistent punish is dash attack. A properly buffered dash attack will hit EVERYONE. And it should be doing it with the inward hitbox to put the opponent back offstage.
The only true combos off dash attack at high percent are into SJP but it'll never kill. And it only does 11% (same as a bthrow which would also get them offstage but let you charge fludd more and not worry about missing and dying)
It does however, get the opponent back offstage and set up for bair 50/50 which would kill quite a bit earlier than a bthrow from the roll point would. (though not much from a usmash)
Also at mid percents (50-90) it does true combo into bair/uair much better than dthrow so it's probably best to just usually go for it unless bthrow would kill or reliably set up for an edgeguard.
In Summary:
Get opponent to grab ledge at all costs, roll to ledge and hold shield.
React to ledge jump with uair and combo off it. (Kill confirm with dair/sjp) Prepare to always react to jump if you don't plan on trumping since timing is strictest.
React to standard get-up and ledge attacks with grab and dthrow for dtilt combos or bthrow for kills.
React to rolls with grab at low percent. Dash attack at mid percent. High percents if they have a good ledge roll/bad air dodge/recovery. Usmash if you expected beforehand or it's an average or worse ledge roll.
Punish attempts to hang with trump > bair. If you screw up and they air dodge, don't keep bairing. Land and fludd to keep them from landing and try to punish their lack of invincibility.
Punish rising ledge attacks OoS with usmash/dsmash depending on their position, or if they can attack your shield and still pressure the ledge with mix-ups (like mk, sheik, villager, pikachu, etc) Then roll away and prepare fludd if it's not already charged. At minimum, prepare to space and time your dash attack for the stage spike hitbox if they won't let you get the dsmash/fsmash spacing. You want to be ready to cover a potential high recovery as well as their ledge grab. It's not ideal since ledge spike doesn't kill very well until very high and it's techable but it's the only way to cover all their options.
Finally, for characters able to pressure shield from the ledge via command grabs (Bowser, Ganon, Yoshi, etc) characters with aerial shield breakers (DK, Marth) or characters who can phase through your shield to return to the stage (Fox, Bayonetta, Greninja, Etc) you should feign "fake failed ledge trumps" by caping towards the ledge.
This does multiple things:
1: Makes the opponent scared to hang later since they think you're going for trumps
2: Covers most of these options fairly well. (Haven't tested how consistently)
3: Refreshes your shield
4: Baits out the opponent
5: Turns you around to better punish anything the opponent might try to punish with.
6: Deals damage on hang
Alternatively, stutter step fsmash also works and covers everything but roll. (Also has different timings for coverage, but still good. Best after the opponent is known to be afraid of trumps or takes quick ledge options)
You can't react with this, and it forfeits some advantage if you guess wrong but it's the only way to deter these options.
Disclaimer: This is still a mostly just theory though through actual labbing (not just playing. actually testing and checking if X situation works) But I think this is pretty much the flowchart we should be doing at the ledge almost everytime.
Edit1: You can't react to most ledge rolls oos with usmash. You have to have dropped shield beforehand to usmash, the additional 7 frames for shield drop make it take too long. Grab works though, so you only need to mix it up if bthrow won't kill. Otherwise grab/jump cancel out your drop frames.
DunnoBro have you tested dthrow + frame perfect sh uair combos on fast fallers? I've tested on sheik, falcon and zss and they get comboe'd HARD, it's a small percentage window I don't remember exactly the number but it's around 30-50% depending on the character.
dittos and