So, first up, let's define exactly what triggers Feather Fall:
Situation 1: You can't cast Feather Fall as a reaction to being hit - it's a reaction to falling. If damage caused you to go unconscious, causing you to fall, then you can't use Feather Fall. That's because when you're unconscious, then you're incapacitated, which means you can't use reactions.
Situation 2: Yep, that's exactly how it works. If you land before the spell ends, you take no damage, and you can land on your feet. Taking no damage doesn't depend on landing on your feet, and while the spell gives you the option to land on your feet, while ever you're unconscious, you're prone.
Situation 3: Again, to establish a baseline, the rules on falling:
There's no suggestion of a delay there, so you start falling as soon as you're knocked unconscious. However, we really have no idea how long a fall lasts. You'll have to talk to your DM about that.
Finally, there's the question of whether someone can cast Feather Fall on you when you fall into their range. The reaction for Feather Fall can only be taken when a creature within 60 feet falls, but whether "falls" means "begins falling" or "is falling" isn't particularly clear. I think most people would allow casting Feather Fall on someone falling within 60 feet even if you weren't within 60 feet of them when they started falling, but you'll need to check that one with your DM too.
All that aside, you might be interested to know about the Ring of Feather Fall - it's a magic item that is pretty much designed for this sort of situation. If you can get your hands on one, you never need to worry about falling again.
Yes, but only under very specific circumstances
*Throughout, "you" is assumed to be the creature that is falling attempting to cast the spell on themselves
Unless you prepared an action, you cannot cast earthbind while falling
The rule for falling assumes that a creature immediately drops the entire distance when it falls.
When you start falling, you drop immediately with no time to take an action. That is what makes feather fall so useful, because you can cast it as a reaction in response to falling which does interrupt the fall. Earthbind takes an action to cast and thus cannot be cast while falling.
The only way to get around this would have been to have prepared an action to cast earthbind if you start falling next round.
Falling from >500 feet (optional rule)
Xanathars Guide to Everything provides an optional rule that allows for non-instant falling.
When you fall from a great height, you instantly descend up to 500 feet. If you’re still falling on your next turn, you descend up to 500 feet at the end of that turn.
So, if your DM is using the rule and if you fall from >500 feet you would be able to cast earthbind.
If you want to other Q&As dealing with casting while falling look at the answers for Does casting Fly on a creature who is falling cause fall damage? and Can you cast a spell with a Doss lute while falling?
If you find a way to cast earthbind while falling, you (may) still need to fail a save
The target must succeed on a Strength saving throw or its flying speed (if any) is reduced to 0 feet for the spell’s duration.
Jeremy Crawford says you cannot fail a save intentionally:
No rule lets you opt to fail a save. As DM, I might allow it, assuming you aren't incapacitated or dominated.
So, assuming your DM is abiding by this, you must roll and actually fail a strength save.
If your DM allows failing a save intentionally then this is not an issue.
If you can cast the spell in time and fail the save, you should reach the ground safely
An airborne creature affected by this spell safely descends at 60 feet per round until it reaches the ground or the spell ends.
If you are higher than 600 feet when earthbind is cast then the spell will run out before you reach the ground and you could take damage.
The Rules are Unclear
Your question is reasonable, because Feather Fall's casting time might lead you to conclude that it can only be used at the start of a fall:
The issue at hand is that "fall" has two relevant definitions (as a verb):
If we use use the second definition, then the "fall" only happens "when" the creature begins descending. But if we use the first definition, then someone "falls" whenever they are still descending under the influence of gravity (e.g. "during this round, the creature falls").
The spell has considerably reduced utility if it can only be cast at the start of a fall. For example, since it only lasts a minute, it could not save a creature from a fall of more than 600 feet. And since it has a range of 60 feet, it also could not be used to save a creature that falls from higher than that above a spellcaster. It seems likely that a DM would rule that it can be used during a creature's descent, to stop it from being an extremely niche use spell, but it is up to the individual DM.
Using once you are 60 feet above the ground
Let's assume, for the moment, that your DM permits you to cast Feather Fall during a fall, not just at its start. Your example (casting it when you are within 60 feet of the ground) still may not work as you intended.
If you want to cast the spell when you are within 60 feet of the ground after falling normally for an extended period, you are essentially asking to cast it when you will spend exactly one round falling gently, and then still be able to use your full movement on the ground (an understandable goal). But while you fall, you do not have an accurate readout of their current height at your disposal. And if you have been falling for 500 feet, you are moving very fast (realistically, you will have less than half a second to cast the spell while you are within 60 feet of the ground: and if you are moving 500 feet in 6 seconds, the timing is similar). You might cast the spell too soon, and be more than 60 feet above the ground, or too late (and splat).
A DM could call for any type of check (Perception, for example) with whatever DC they wished if you wanted to attempt a HALO style jump with the Feather Fall spell. Although "rule of cool" could certainly grant you some leeway, you might want to check with your DM before you attempted this tactic.