Technically you fall instead of going prone. You only end up prone if you actually take falling damage, which is fairly unlikely given the fact that you get to fall your fly speed safely.
And it's a good reason to make sure to stay one square up when playing a Pixie. Requires someone to knock you prone twice in order to actually make you prone.
Not true - you can be flying at an altitude of 0, so you're at ground level but not touching the ground, so the first prone grounds you and the second one prones you.