Going by the book, falling damage is always going to cause damage. All you can do to mitigate it is create armor of some sort (the book says by rolling Athletics), but once you're falling, you're going to take some kind of damage.
A block would do, creating the spell as armor is not necessary, I think. My confusion was mainly the distinction between a block against movement vs a block against fall damage.
Think Feather Fall in D&D or slow fall abilities Monk classes get in various RPGs and MMORPGs. Mitigate damage not actually stop falling. Fall...slower.
See above. The mitigate damage spell would, for me, be a block against damage from falling (however it plays out narratively), and I have a hard time wrapping my head around the concept of a block against movement in this circumstance, as UmbraLux put it.
And again, an actual feather fall spell for me would be thaumaturgy. In most cases, you'll end up with 0 stress after the fall anyway, sure. But it plays out in different ways.