Lyxen
Great Old One
It has very specific rules. Misty Step is clear in what it allows. It allows you to appear in an unoccupied space. It does not allow you to angle how you come out,
Positioning your body is not even an action in 5e, it can therefore very well, using RAW, be considered a "flourish" (Your turn can include a variety of flourishes that require neither your action nor your move.), and dropping prone does not use any of your speed. So here you are, it's RAW that you can appear in almost any position, what it can not do is put you in a position where you stand upright if you were not that way.
because that is not written. It does not allow a fall to end, because that is not written.
You are only falling if there is nothing under you. If you Misty Step to where a place where there is something under you, you are not falling. This is perfectly RAW. And the RAW says nothing about momentum being conserved, if you think it's there, please tell me where it is.
Actually, RAW, it's you who are wrong, according to the falling rules: "At the end of a fall, a creature takes 1d6 bludgeoning damage for every 10 feet it fell, to a maximum of 20d6." It's not the falling that causes damage, it's the hitting at the end. So if you are falling, do not hit anything, and then you misty step to a position where you are not falling, you do not take damage. You are not falling after the misty step, the fall ended before you hit the bottom, that's all.
There are also the instantaneous falling rules, which stop a bonus action from working at 500 feet or less.
But as I demonstrated before, using a readied misty step as a reaction (with a trigger 20 feet from the ground) interrupts what is happening, i.e. the fall. So you misty step, and when you appear, you are not falling, so the fall does not resume, actually it ended mid-air without you touching anything and therefore without you taking any damage.