I see. You don't want to imitate the behaviour of having no collision with only units, but retaining it with other objects and cliffs. It's sort of possible to do, but takes an extremely large amount of time, effort, and it will still be far from seamless (it's basically just bad). You definitely want to stick with the Wind Walk method.
As for retaining the effect of the ability after casting something, that's doable. It's going to have one side effect which you might or might not like.
- Keep track of the duration of your Wind Walk-based ability in a variable.
- Keep track of its cooldown.
- Have a periodic check running to see if the buff got removed prematurely by another ability being cast.
- If this happens, finish the cooldown of Wind Walk, set its mana cost to 0, make the unit cast it.
- Immediately set the mana cost back to what it should be, and start the ability's cooldown over as many seconds as there was left, since you're keeping track of that too.
The side effect is only visual; the cooldown graphic (that light blue tint over the button) will appear as if Wind Walk's cooldown just started from its full duration.