I have previously air-shot enemies while airborne. It is possible - unlikely, but possible. It might actually happen accidentally in a confined area if you both splash on the ground while jumping. .
Very easily, actually. Consider you rocket jump at the same time as someone else, both with the direct hit, all you have to do is fire at the same time and it's instant death for both of you. So considering you both have the direct hit, it's very easy.
If you look at the **** that happens in games, yes, it's definitely possible seeing how many matches are played everyday, though the chance is of course still small
the soldier class is basically built to rocket jump, all you need to do is hit someone who also hits you, while you are both in the air. not easy, but perfectly doable, even by accident.