A CR 3, fine swarm has *no* place in something that was designed to be played by a party of 4 brand new PCs or pregens. Even assuming they had grenades or spells, 40 hp means that the party has to somehow deal 28 damage. That just isn't reasonable.
Then a CR *6* encounter? Against 2 creatures that are closer to CR 5 than CR 4? With confusion, in a tight space?
No, even with the Deus ex Machina rescue, that was completely unreasonable. And a Deus Ex as someones first experience of organized play is a sure fire way to ensure they feel like their character is useless, non viable, and never come back.