[RPG] If you misty step 30 feet straight up do you have time to attack a foe next to you before you start falling

Xanather's Guide to Everything now clarifies this in two ways. Firstly, it provides an explicit explanation of the RAW and provides an optional rule for long falls.

RAW - Immediate fall & immediate damage

The rule for falling assumes that a creature immediately drops the entire distance when it falls. (XGtE)

So by standard rules the answer to your questions are:

1. Alice starts falling on Bob's turn immediately after the trigger that causes her to fall
2. She immediately falls the entire distance and takes the damage (barring some intervention that halts her fall).

As for your follow up questions:

• Is a reaction (e.g. casting Feather Fall) on Bob's turn the only hope?
  • That or a readied action are about the only ones I can think of right now
• Can Alice's ally Charlie swoop in on his subsequent turn within the same round to catch her?
  • No, she has already fallen
• Can Alice spend half her movement on her next turn to "stand from prone" and pull out of her involuntary dive?
  • No, she has already fallen
• If Alice had expected to be shot down, could she have spent an action to be Ready to "stand from prone" while moving through Bob's airspace?
  • Yes, you can prepare an action this way to use half your movement to stand up from prone.

Optional RAW - 500ft/turn & End of her next turn

However, XGtE also provides an optional rule for long falls that says:

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. This process continues until the fall ends, either because you hit the ground or the fall is otherwise halted. (XGtE)

In this case, the answers to your questions are:

1. Alice starts falling on Bob's turn immediately after the trigger that causes her to fall (same as in the RAW case).
2. She falls 500 feet instantly which still leaves her 105 feet in the air. On the end of her next turn she will fall the rest of the distance if she does nothing, but allows her to presumably use her turn to take preventative action if she so desired and was able.

As for your follow up questions:

• Is a reaction (e.g. casting Feather Fall) on Bob's turn the only hope?
  • No since she will take her next turn (as would anybody else who was there) before hitting the ground
• Can Alice's ally Charlie swoop in on his subsequent turn within the same round to catch her?
  • Yes! Though the mechanics of how this would actually work would be up to the DM as catching a falling person is going to not be an easy task!
• Can Alice spend half her movement on her next turn to "stand from prone" and pull out of her involuntary dive?
  • Yes! At the beginning of Alice's turn per:

    But if that creature starts any of its later turns still falling and is prone, it can halt the fall on its turn by spending half its flying speed to counter the prone condition (as if it were standing up in midair). (XGtE)

• If Alice had expected to be shot down, could she have spent an action to be Ready to "stand from prone" while moving through Bob's airspace?
  • Yes you can. Same as with the standard RAW.

Which method is best for your group will be up to your DM, but personally, I find the optional rule to be much more intuitive and reasonable an answer at my table.

So, first up, let's define exactly what triggers Feather Fall:

1 reaction, which you take when you or a creature within 60 feet of you falls

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:

If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic, such as by the fly spell.

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.