Never ridden a fat bike, but the physics there seems a bit off.
For the gyroscopic effect, I thought the force required was directly proportional to the mass (could be wrong). So increase the mass by twice as much makes the force required double. Which isn’t a huge amount. Also I thought the force was proportional to rotations per second, so there shouldn’t be a leap where you’re fine at 19mph, but doomed at 20mph.
It’s been a while since I’ve done this sort of stuff though, and we never went that in depth.
As for the tyre being pushed away from the rim and destroying the fork at high speed, that sounds like a dodgy tyre or pressure set up. Remember, it’s very similar to a normal bike, just scaled upwards a bit.