Knockback against opponents with Flight, Clinging, Catfall

[nerfninja]

Hello, I'm starting a campaign shortly and one of my character's primary attacks is a massive knockback attack. Specifically, it does 2d x 10 (No Wounding, No Blunt Trauma, Double Knockback). If I did my math right, I can potentially send an elephant flying 5 yards or a standard goon around 28 yards.

I was planning to do some playtesting to get a feel for knockback and collision rules, and some interesting questions came up.

If I understand correctly, if the attack is not successfully defended, then my knockback damage applies and there is no resistance roll whatsoever (Go Flying Goon!). Opponents suffering knockback test to see if they fall down at a penalty for each yard of knockback beyond the first. If they collide with anything in the way, they will suffer collision damage.

If the knockback does not bring them into contact with any terrain, do they suffer collision damage with the ground? Any suggestions for my GM on how to calculate that if so?

What if the opponent has an advantage like Flight, Clinging, or Catfall. Can these powers be used to mitigate collision damage or reduce knockback distance? Our campaign involves enough fantasy martial arts and Powers type abilities for me to expect to encounter a situation like this.