This Dair offstage is day 1 stuff. No really, everything you put in your Reddit write-up was found Day 1. Some of it was known even before Corrin's release. Dair offstage is super telegraphed and easy to avoid if the other player understands their recovery, barring very specific circumstances. That's why it's not used. They'll just avoid it and then you lost stage control. And the only risk involved is that of a gimp. Almost nobody goes for gimps. Therefor it has almost no risk. The risk/reward on Dair offstage is hilariously skewed against most players (players, not characters) yet it's still not used because you need a miracle for it to hit. It has nothing to do with tournament players not knowing about it. It's just a poor option overall.
We also still have basically no idea of how Dair works anyway, so lab that out if you want. How does it trade? Why can you sometimes footstool out of Dair offstage and sometimes not? Which recoveries will the disjoint beat out consistently? Stuff like that.
Although in certain match ups, like against Falcon, it can sometimes work to do a full hop near the ledge to bait out a high recovery, then fast-fall to the stage and F-smash him or something. Don't expect this to work consistently though.
*************
Moving away from Dair, I recently labbed a new edgeguarding option I thought up. My labbing was minimal and I'll work on it more when I get the time. I doubt I'm the first one to think of this, but I've literally never seen this anywhere.
It's run offstage -> DFS. DFS is small shot big bite, of course.
What I like about this is that it's basically unreactable in most situations, so an airdodge is highly unlikely. The angle that you fall when running off the stage is about 45 degrees downward. I've been using at ~Mario's up-b distance, maybe a pinch closer. If possible, it's good to use it as soon as the opponent's up-b starts.
It works REALLY well against Falcon. I got it almost every time in training mode. I'll assume it works on Ganon too. It works quite well on Roy, though a counter may sometimes be better against him. I've tried it on other diagonal recoveries like Mario and I found that his up-b is hard to hit, so you may want to use DFS before he starts it.
I suspect it SHOULD work on horizontal recoveries like Fox or Ike side-b, but I'll need someone else to test that.
Additionally, you can use it at any point during an opponent's recovery, although you may have to read an airdodge in certain situations.
************
Also I'm staring to prefer B-throw over F-throw. It feels like it has less endlag (though I don't have the frame data for that). It feels easier to pressure after B-throw than after F-throw, though I'll work on it more when I get the chance.