Related to these questions:

Can a druid wild shape into a cranium rat and use telepathy?

Can a Druid Wild Shape into a Swarm or “Giant”?

The Cranium Rat question establishes that the full stat block is used, and the Swarm or Giant question clarifies that you can only turn into one creature, per the spell.

Could 18 druids (I'd say 18, because one cranium rat has 2 HP and the Swarm has 36 HP per the stat block in DnD Beyond) wild-shape into 18 Cranium Rats and form a swarm, per the description of the Cranium Rat, and gain all the additional benefits that a Cranium Rat Swarm has?

The description says:

Evil Collectives: Cranium rats are no smarter than ordinary rats and
behave as such. However, if enough cranium rats come together to form a
swarm, they merge their minds into a single intelligence with the
accumulated memories of all the swarms constituents. The rats become
smarter as a result, and they retain their heightened intelligence for
as long as the swarm persists. The swarm also awakens latent psionic
abilities implanted within each cranium rat by its mind flayer
creators, bestowing upon the swarm psionic powers similar to spells.

Also, the Cranium Rat Swarm's Innate Spellcasting (Psionics) says:

The swarm's innate spellcasting ability is Intelligence (spell save DC
13). As long as it has more than half of its hit points remaining, the
swarm can innately cast the following spells, requiring no components:
At will: command, comprehend languages, detect thoughts 1/day each:
confusion, dominate monster

Now, the Druid Wild Shape feature says you can't cast spells, but the description of the Cranium Rat says:

The swarm also awakens latent psionic abilities implanted within each cranium rat by its mind flayer creators, bestowing upon the swarm psionic powers similar to spells.

So this means that the 'spells' are NOT in fact actually spells, but are instead psionic powers, even though the stat block does call them 'spells'.

Does this mean that such a collective of Druids get access to these powers as well if they Wild Shape into Cranium Rats and form a swarm?

Side Note: It would be an interesting story element to have a group of Circle of Decay druids using this ability to combine their knowledge and experience periodically, and expand their power by teaching each other new skills, magic, and other things. Especially if the GM rules that the stats for the Wild Shaped druids make a much more powerful 'swarm' by scaling the swarm Cha, Int, and Wis stats based on the druid's stats (Since Wild Shape lets you keep those stats when in Wild Shape) the same way they are scaled up from the Cranium Rat to the Cranium Rat swarm.

It's up to your DM.

We don't have any rules about what has to be true for a cranium rat swarm to form.

We don't know if there are checks involved, we don't know how long it takes, we don't know if the cranium rats have to be mentally compatible, and in particular we don't know how many cranium rats it takes to form a swarm. (You've suggested that 18 cranium rats should be correct, and you derived that number by dividing the hit points of the swarm by the hit points of a rat, but that is your house rule and not necessarily correct.)

It would be an error for us to say "yes you can do this", because someone might then point their DM to the answer and say: "look, rpg.stackexchange says you have to allow this!" and we shouldn't be issuing house rules for you.

I do think this would be a cool thing to have happen, but it would have to be done with the DM's cooperation anyway since you're unlikely to have 18 player character druids in the same campaign.

Probably Not

The nature of swarms is defined in the Monster Manual:

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. (MM, p. 337, bold added)

The commonality to all the examples given is that a swarm is a group of animals that is influenced by a single malign intelligence. Cranium rats fit this mold quite well also, although in their case the single malign intelligence is formed from their collective minds merging into one:

Cranium rats are no smarter than ordinary rats and behave as such. However, if enough cranium rats come together to form a swarm, they merge their minds into a single intelligence with the accumulated memories of all the swarm's constituents. The rats become smarter as a result, and they retain their heightened intelligence for as long as the swarm persists.(Volo's Guide to Monsters, p. 133)

So the very thing that allows cranium rats to act as a swarm (unlike mundane rats who aren't under the influence of some other creature) is that their minds all alter to become part of a single collective mind, and become more intelligent as a result of their shared consciousness. But a central feature of wild shape is that using it does not alter the intelligence or consciousness of the Druid. This is reflected in the description of wild shape:

Your game statistics are replaced by the statistics of the beast, but you retain your alignment, personality, and Intelligence, Wisdom, and Charisma scores. (PHB, p. 67)

The wild shaped druids could definitely gain mental abilities that they usually don't have (e.g. the cranium rat's telepathy), but any feature of the cranium rat that alter's the creature's intelligence will not work on the druid, since "you retain your... Intelligence." And it sounds like this altered intelligence is precisely what allows cranium rats to act as a swarm.

The group of druids definitely couldn't cast psionic spells

You claimed that the cranium rat swarm's psionic spells are not spells. They are.

the swarm can innately cast the following spells, requiring no components:

Other text describing them as "similar to spells" does not override the fact that to use this feature, the group of druids would have to "cast" a "spell." And unless the druids are 18th level or higher, when you are in wild shape:

You can’t cast spells (PHB, p. 67)

Jeremy Crawford also specified this in no uncertain terms.

Wild Shape says you can't cast spells. The feature doesn't care where those spells come from. You can't cast them.

The Rules Don't Support a swarm of Druids very well

Most of what I said above (other than the spellcasting part) is a logical argument that makes sense, but isn't definitive. You could argue that the cranium rat's communal mind feature is a case of "specific overrides general," and although druids don't usually change their intelligence when they are wild shaped, they would in this case (I believe this would be mistaken, but it is supportable). However, making this argument would mean you'd have to fundamentally rewrite several rules regarding swarms.

As a simple example, consider the fact that swarms have no rules for individual members of their swarm running out of hit points. You do damage to a swarm as a collective, and although damage does kill members of the swarm this is abstracted rather than precisely calculated (mainly abstracted by the fact that a swarm with half hit points does less damage when attacking). However, wild shaped druids change back into their natural form when they lose their wild shaped form's hit points. So when does this happen to the druids in the hypothetical swarm? Does it happen gradually, every time the swarm takes 2 hit points? Or does it only happen when the entire swarm's hit points are exhausted?

Similarly, if you rule that the wild shaped druids in cranium rat forms meld their intelligence when they are together, you should also similarly rule that a single druid wild shaped into a cranium rat who is around other cranium rats loses his or her individual consciousness, and instead becomes part of the cranium rat swarm's collective mind. Also, you should also cause them to be entirely controlled by any nearby mind flayer colony's elder brain, since this is also a feature of the cranium rat's unusual method of thinking. If this was your ruling as a DM, it might result in a PC becoming an NPC (possibly permanently) by virtue of them simply being in the vicinity of other creatures, which seems a dangerous ruling to make.

To sum up...

At the end of the day, the rules leave some wiggle room for you to pursue this idea, and the "rule of cool" definitely gives it some fascinating story possibilities (druids who form a bizarre elder brain like collective consciousness, and begin a fully new form of life in their collective community): but it contradicts many available rules, and directly goes against the intended nature of wild shape. There are many beasts whose minds are fundamentally different from those of humanoids: from the instinct driven lives of insects to the practically memoryless life of certain fish. But druids wild shaped into the form of these animals do not lose their memory or consciousness when they look like these beasts: and for the same reasons, they should not change their way of thinking to fit that of the alien and other mind of the cranium rat.