"Finally spot the cave's back wall forty feet ahead of them" (OOC clarification, the right hand passage with the bandits feeding the frogs is a dead end correct?)

As far as you can see from where you stand, yes. There may be further passages to the left of the pool, but because of the cavern wall, you cannot tell without getting closer to the bandits. Here is a map of Croaker Cave as you have seen so far.

"Where would we get bats from?"  Carina is genuinely puzzled at Taiga's suggestion for a moment until she has a sudden hunch.

"Unless... if there is something non-evil producing that foul smell in the left (eastern) cavern, might it be... bats?  Or bat poop?"

She has no earthly idea how to tell, but she looks helplessly at Elbert, in case the ranger knows how to recognize animals by the aroma of their feces...

"I am still game to just try hailing them, saying we know of Hulil's condition, and convincing them we are here to help.  I like the idea that we tell them their five compatriots are with Elro now, but that we are focused on limiting the spread of the disease and finding a cure, not punishing bandits."

She looks around.  "But again, I am an outlander here and will go with the group decision."

“I don’t know where there are bats, but these people found some, “ Taiga reasons. “But then, I think you’re right Carina,” Taiga says pensively, “it might be best to talk to them. If we bypass everyone and the result is that we have to fight all at once in the end, perhaps it is best to feel out the ground and see if they are hostile on sight. Because if they are, it is better to cull them in small batches.”

Always looking to avoid a fight Brix says, "Let's check out the smelly cave. I would like to know what's behind me. Even if we encounter resistance and it br8ngs these ones into the fight at least it won't be a surprise and maybe we can keep from being surrounded."

Elbert will attempt to identify the foul smell.

Nature: 17

The party of adventurers, in weighing their options on how to proceed through Croaker Cave, decide to return to the foul-smelling cavern beside the previous pool. Elbert approaches the entrance of the dark cavern, the absence of light inside pressing on his eyes like a thick cloak so he cannot see within — but now that he is closer, the Ranger is fairly certain the smell is coming from animal feces. Whether the waste of bats, or giant frogs, or something else entirely, Elbert isn't sure. 

Taiga helps Elbert to identify the smell, knowing that she can see better in the dark, even though she still holds the torch, too. “We shouldn’t separate too much from the group,” she whispers preoccupied. 

Perception: 21, (20 if at disadvantage)

Nature: 22

Taiga follows the Ranger to the entrance of the foul-smelling cavern, holding up her torch to cut through the shadowy blackness. As she does, bright eyes blink their way into sight — peering from the stalactites of the cave like pinpricks in the darkness. And all at once, a swarm of bats descends from the cavern ceiling and flaps straight towards the party of adventurers. Everybody, go ahead and roll for initiative!

Initiative: 16

Ragnus hears the flapping and looks towards the sound. He grumbles to himself, "Here we go...."

((Sorry! This one is on me!))

"Here are our bats, and they come conveniently to us," Taiga mutters as she lowers the torch to the floor to get her hand free for her rapier. 

Initiative: 6

Initiative: 21

Carina Initiative:  19

Brix: 12

As the swarm of bats descend upon the party of adventurers, angered by the torchlight that set their dark cavern aglow — Elbert will be the first to react, with Carina on deck. Elbert, what will you do?

OOC:  Just in preparation for Carina's turn after Elberts:  has the swarm of bats approached within 5' of some or all of the characters yet?  Or can we still make ranged attacks without disadvantage?

Looks like we're doing theater of the mind, which is perfectly good, but need to know the above to decide whether to use her warhammer or Eldritch Blast.

If there is an enemy within Elbert’s 10’ reach then he will attack with his glaive followed by a bonus action attack with the opposite end of the polearm.

If no enemies are in range he will remain at the entrance to the cavern and ready an action to attack the first one that comes within 10’

Attack: 11 Damage: Unable to parse dice roll.

Bonus Attack: 8 Damage: Unable to parse dice roll.

Weird…

Damage rolls: 11, 7

Omg 🤦

(I'll post for Carina as if the bats have not closed to within melee yet so she can safely back up and make ranged spell attacks without disadvantage).

Carina backs up 30' as quietly as she can and her eyes strobe black ever so briefly.  A black beam of energy which seems to originate at the dark harmonica around her throat and resonate with the black warhammer at her belt shoots from her fingertips towards the bats, hitting a few of them.

Carina's Eldritch Blast to hit:  24
Carina's Eldritch Blast damage:  3 (force damage) - ugh, both Carina and Elbert with the low damage rolls.