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.
Flail Snail and Invisible Stalker are both valid choices for conjure elemental
Choose an area of air, earth, fire, or water that fills a 10-foot cube within range. An elemental of challenge rating 5 or lower appropriate to the area you chose appears ...
Firstly, the Flail Snail is CR 3 so it is a valid choice for this spell. The Invisible Stalker is CR 6, which would be invalid, but the spell can be upcast to increase the CR range, so if the spell is cast with a 6th level spell slot or higher, the Invisible Stalker then becomes a valid choice too.
From the PHB, pg. 225:
At Higher Levels. When you cast this spell using a spell slot of 6th level or higher, the challenge rating increases by 1 for each slot level above 5th.
Secondly, both of these creatures are "elementals" (which is one of the creature types like "humanoid" or "beast"); this is what the spell is referring to when it says:
An elemental of challenge rating 5 or lower
not just those that are called "Elemental" such as "Fire Elemental", etc.
This creature type is outlined in the Monster Manual, pg. 6:
Elementals are creatures native to the elemental planes. Some creatures of this type are little more than animate masses of their respective elements, including the creatures simply called elementals. Others have biological forms infused with elemental energy. The races of genies, including djinn and efreet, form the most important civilizations on the elemental planes. Other elemental creatures include azers, invisible stalkers, and water weirds.
Also, both of the creatures you mentioned are each associated with one of the four elements. Flail Snails are associated with earth (Volo's Guide to Monsters, pg. 144):
A flail snail is a creature of elemental earth ...
And Invisible Stalkers are associated with air (Monster Manual, pg. 192):
An invisible stalker is an air elemental ...
Best Answer
Your reading is correct.
When casting Summon Lesser Demon, you have to roll a d6: if you obtain a 1 or 2, the table says that the spell summons exactly 2 demons of CR 1 or lower. Since manes' CR is 1/8, and 1/8 is less than 1, the DM may make the spell summon 2 manes, per description:
One may find more information in the Sage Advice Compendium about DM choosing the creatures summoned by a spell once the CR has been established: