Unfortunately, Mike Mearls is not the proper authority on these matters - although he does try his best to help. Since he openly admitted he could not recall the rules on the matter and gave an educated guess, Jeremy Crawford recalled them for him.
As per the intended RAW in PHB pg. 191, under "Moving Around Other Creatures":
"...you can’t willingly end your move in its space..."
Further explained by Jeremy Crawford in the tweet linked above:
"...you can't willingly end your move —any part of it— in another creature's space..."
So let's first clear up some confusion as to the phrasing of this rule.
- We know each creature gets a turn (1 turn) in the initiative each round; which starts being explained on PHB pg. 189.
- We know each creature may take 1 action (Attack action, Cast a Spell action, Ready action) and possibly 1 bonus action (there are several) during that turn, with details also beginning on PHB pg. 189.
- We know from PHB pg. 190 that you may also move an equal distance up to your speed, and can use as much or as little of your speed as you wish as well as breaking that movement in between various forms of travel (walking, climbing, jumping, etc) and that there is no 'Move Action'.
- We know from the next section on PHB pg. 190 that you can break up your movement in between attacks and other actions.
With what we know from the above clearly written rules, it then becomes clear that when Crawford says you can not end any part of your move (as detailed above) in a creature's occupied space, he is referring to your Speed-based movement, and is not solely referring to ending your turn as a whole, which can include any of the above listed. Your only choice is finishing your movement in an unoccupied space near the creature in question, attack your target of choice, then move back through (if you have movement remaining) the occupied space making sure you (once again) do not end any part of your movement in that creature occupied space. Then you can end your turn. Keep in mind that each time you move through the space, it is difficult terrain, and costs you extra movement unless you have the ability to ignore difficult terrain.
With this in mind... the only relevant question you have remaining perplexes me, and I think there are only 2 outcomes.
- If Alice tries to move through an ally's space, but she is stopped by a Ready-action grapple from a hostile creature near the ally's space, then the Ready action you had in mind triggers (I am assuming?) the moment Alice is in range, interrupting her movement while she is still in her ally's space and effectively reducing her speed to 0, as per the Grappled condition.
Her movement has just been forcefully (unwillingly) stopped in another creature's space.
- Alice tries to somehow move through a hostile creature's space but they Ready a grapple. In this case the Ready-action grapple would trigger when she got within attack range, because a grapple is always dependent on the Attack action, which you must forgo in place of a grapple. An enemy could never Ready a grapple to trigger when Alice steps into or onto their space.
- There are no rules that apply any penalty for this outcome.
- The space is difficult terrain anyway, so the addition of Alice to the space changes nothing.
- We know a 5-foot 'space' is generally 5 feet of 'effective creature threat', and not a humanoid/creature 5 feet wide. So roughly 10 humanoids could stand grappled together in a 5-foot space, which is 25 square feet.
- Stacking is another story entirely, and then you are dealing with distance/height in feet versus the creature's speed (which becomes 0 and so couldn't be tossed, thrown, or otherwise put on top of a grapple 'pile' higher than 1 foot tall).