rapid acceleration as much as motor could do
The motor needs to do TWO things - accelerate per your instruction, AND try to keep you upright. When you do "rapid acceleration as much as the motor can do", you've left nothing in the tank for it to help balance you with.
Gentle acceleration. Always. I know you think the OW "locked up" on you, and I don't want to say that's impossible (software or hardware failure is always possible), but IMO it's far more likely that you simply overwhelmed the board's ability to do both things at once that it needed to do, and forced your nose down to ground.
The instant that happens (without nose wheels like Fangs or similar), yes, the board stops immediately, and you get thrown. But not because the wheel locked up on you. If the board was still on afterwards (and you say it was) what I describe seems far more likely to me.
I know I'm not telling you what you want to hear. But I had what I think was a very similar accident at 250 miles. I messed up my shoulder bad and it's still not 100% healed over a year later. (I got back on the horse as soon as I could though).
I do agree that pushback is not very useful in this scenario (overacceleration). It's much more useful when you're already cruising and nearing max safe speed and you can use it as a cue to back off.
But the board is not magic - you outweigh it, by a LOT, and can easily overleverage it - and in an overacceleration scenario (even well below 15 MPH) you can push past pushback faster than pushback can push back (say THAT ten times fast). You'll never feel pushback, or even if you do you'll have no time to react.
Accelerate gently/smoothly, always. Bring your front foot back toward the wheel. Make sure you always keep your weight centered above the wheel, or even just behind it. Don't "lean" to go; instead press down the relevant leg and lift the other, while still keeping your weight centered above wheel.
The margins between "OK" and "Not-OK" are very, very thin on a one-wheeled board.
Heal up soon!