Reporting back on the Donguri combo video, I finally figured out how he did the ground dashing heavy attack in the air. The timing is pretty strict. Think of it like FRC from Guilty Gear. Thankfully it can be performed without a target so practice can make perfect.
Basically, you want to queue up a weapon swap during a launcher, after which, perform an evade. The queued weapon swap will automatically come through during the evade. You want to hold forward (can be done early) then attack with heavy basically an instant (Unable to identify how many frames) before the weapon swap, or might even be exactly on it. That will result in the ground dash attack of the offset heavy to come out in the air. Make sure to not hold/release the weapon swap and evade button after you input or it will not work. It was kind of obvious from the video as you can tell when the weapon swap and evade are queued, but it was hard to know whether that was all there is to it or if something else was missing. It took a couple of hours to finally get it once, then it became fairly clear what the timing is.
After practicing for an half hour at it, I think it's not ridiculous enough where it cannot be done consistently (90% success rate at this point). It certainly is no harder than a FRC. As far as for how practical it is, I think it's fairly ideal if your launcher catches multiple targets, since those attacks have more coverage than standard attacks in the air. Downside is obviously you have to sacrifice your evade to do it, so you will be vulnerable in the air, although I'm not sure how much that matters. That would depend on enemy types in the main game, so people that are actually playing the release will have a better idea.
Hopefully there are other interesting quarks with how the combat functions in the game. The way things behave when input queue is involved is pretty interesting.
Edit: Added another condition that I forgot to the inputs.