If you break concentration it becomes hostile and might attack you
From the invisible stalker entry under Directed Hunter, "...forced to serve until the magic that binds it expires". The magic that binds it is Conjure Elemental and when you lose concentration that magic expires.
Then we go to Conjure Elemental which says, "...you lose control of the elemental, it becomes hostile...and it might attack", so that happens.
If you die it becomes hostile, and could attack you, but instead just finishes its task and leaves
In a nice case of specific beats general the invisible stalker entry says, "If the summoner dies in the interim, the invisible stalker vanishes after completing its task". So if you manage to die without first breaking concentration for another reason (say from massive damage) the invisible stalker will finish what you asked of it.
No; conjure animals summons a specified number of beasts of a certain CR, not a swarm of them
As written, the conjure animals spell summons a specified number of beasts, and a swarm is a collection of an indeterminate number of creatures. As the Monster Manual specifies (p. 337-339), each of the swarms listed there is a "Medium swarm of Tiny beasts"; a swarm is not a single "beast". The creatures within a swarm are not treated as distinct entities when treating the collective group as a swarm.
Thus, a swarm of any sort of animal would not qualify for any of the four options listed in the conjure animals spell.
Furthermore, the "The Nature of Swarms" sidebar in Appendix A (MM, p. 337) clarifies what swarms really represent:
The swarms presented here aren’t ordinary or benign assemblies of
little creatures. They form as a result of some sinister or
unwholesome influence. A vampire can summon swarms of bats and rats
from the darkest corners of the night, while the very presence of a
mummy lord can cause scarab beetles to boil up from the sand-filled
depths of its tomb. A hag might have the power to turn swarms of
ravens against her enemies, while a yuan-ti abomination might have
swarms of poisonous snakes slithering in its wake. Even druids can’t
charm these swarms, and their aggressiveness is borderline unnatural.
It's clear from the rules that swarms aren't designed to be treated like mundane animals, and that they don't simply congregate naturally or for benign reasons. Rather, they're a result of some malevolent influence, disturbing the natural order of things in some way.
The designers have been remarkably consistent in their interpretations of the rules regarding swarms: they are not a single beast.
As of October 2020, this question is explicitly addressed by an official ruling in the Sage Advice Compendium:
Can conjure animals summon a swarm?
No. Conjure animals summons individual creatures, and swarms are
groups of creatures.
This official ruling is simple and straightforward, and matches the rules referenced above.
It also lines up with a number of unofficial rulings on Twitter about using Wild Shape to turn into a swarm - by Jeremy Crawford in January 2016 and again in November 2016, and by Chris Perkins in June 2015 - as well as an unofficial ruling on Twitter by Crawford in January 2015 about whether a swarm of quippers counts as one fish for the purpose of the trident of fish command (which can be used to cast dominate beast on "a beast that has an innate swimming speed").
As you can see, the rules themselves, the official ruling in the Sage Advice Compendium, and a number of unofficial rulings on Twitter all indicate that a swarm is not "one beast", but rather a collection of beasts.
Best Answer
The DM Decides
Taking from Purple Monkey's answer to the linked question:
From the Sage Advice compendium:
Conjure minor elementals is explicitly listed as an example of the sort of summoning spell where the DM is the one to decide which creatures appear.
Technically, the DM doesn't even have to let you have any say over what element the creature is related to, since it says (PHB, p. 226):
as opposed to conjure elemental (PHB, p. 225), which says:
and therefore the element of the creature is determined by where you cast the spell, for conjure elemental. Since conjure minor elemental just lets you summon it anywhere, the DM gets to decide the element type too.