Oh yeah, that Nair would wreck you too lol, so definitely be holding to the side if doing A. Oh, and now that I think of it, pressing B would also be bad even without pressing to the side, because it would bring up the Monado Arts, preventing you from doing your up B. Pressing the grab button isn't a bad solution, though, causing an air dodge. However, if you're used to the timing of the delay between the two strikes, the grab button might mess up your timing if it on the shoulder of the controller.
Hmm. There's only a few situations I could see this coming in use. One of them is at very low percents. Press to get to Buster, grab, U-throw, Buster activates, U-tilt, U-tilt. At very low percents, Monado Buster's U-throw does not send opponents high enough, so this might be good to overcome that. Other situations I could see are to U-throw before Monado Smash activates so that the enemy does not go too high then air slash. Killed Pikachu at 90% when I tried it in training mode. Other than that, maybe throw the opponent off the stage before Smash activates to have them in a closer range? I'm not too sure if you could benefit from that at all.
However... this just made me realize something I think is pretty cool. Remember how @
Masonomace
found you can deactive Monado Arts while doing something else like shielding or attacking? That can apply here! Deactivate Buster while doing a B-throw -> Back Slash combo to get an early KO depending on location on the stage. Testing this in practice mode, it KOs Mario if initially at 58% if he lands at the far edge of final destination. If Mario is initially at the middle of FD it KOs at 61%. A pummel in Buster mode does 4% damage, so technically those percents could be a bit earlier. Honestly, this seems completely viable.