Falling is a free round action.
"a flying creature fails to maintain its minimum forward speed, it must land at the end of its movement.
If it is too high above the ground to land, it falls straight down, descending 150 feet in the first round of falling. If this distance brings it to the ground, it takes falling damage."
If I choose not to move, I fall 150 feet, apparently.
I can choose to turn off flying at any time. Lets assume I have wings from grafts. I stop flapping them. I drop the 150 feet as a free action. Now, Slow fall says nothing about anything ELSE taking no damage. It only talks about ME, and at 20th level, I take no damage, regardless of distance.
As a matter of fact, It appears that if I land in an occupied square, my target takes damage, no save, no AC roll, nothing. The only protection would be the ASA impact.
Now then...
"Accidentally Ending Movement in an Illegal Space
Sometimes a character ends its movement while moving through a space where it’s not allowed to stop. When that happens, put your miniature in the last legal position you occupied, or the closest legal position, if there’s a legal position that’s closer. "
So, we must assume that my monk, if he gets up 150 feet and chooses to fall, will drop 150 feet in the first round as a free action. Whatever square he is over he hits. Whatever is in that square takes damage from getting hit with a heavy falling object. In this case I am 200 lbs (approx) so it's 16d6 damage.
As long as there was a wall in the last 5 feet, I take no damage. Hell, the way it reads, anywhere along the 150 drop I just need something that is sturdy and I take no damage. So If I'm hanging from a tree branch 150 feet up, I take no damage from the fall.
I cannot end a movement in an illegal square, so I move to the nearest legal square.
I understand your desire to make this make "sense" and to house rule something to make it make "sense" but this is about Power Gaming. Kinda sick and tired of "monkday" over on GitP and I plan on showing them exactly what a monk can do.
A 16d6 Flying Suplex is just one of my many many many devious RAW Monk Theoretical Optimizations.
MUHAHAHAHAHAHAHAHA!!!!