Thanks.
Though you could have made it much simpler to respond by explaining that everything revolves around
I see nothing egregious about this. The effect is appropriate for the level.
As I understand it, your described outcome required the character to make two saves, first rolling a critical failure and then a failure.
Even with the use of a Hero Point, this can happen.
What seems to be a mistake, however, is
"while monsters (immune to the hazard) entered the fray, swarming and dealing sneak attack damage and others at range blasting with spells."
Unless I am mistaken, you have activated room C7 and its monsters while the heroes were still busy with the trap in C6, yes?
This is a mistake. An honest mistake that's very easy to make. But still - encounters are never intended to mix together.
Pathfinder 2's encounter math simply does not allow it. One Moderate encounter can be deadly as is (as you have discovered!)
Two moderate encounters combined... that's immediately equal to an Extreme encounter!
Edit: ninja'd!
Twice as many monsters is quadruple the danger in this game, to phrase it a bit crudely but very directly!
I heartily recommend every PF2 Gamesmaster to
never combine encounters. You simply can't.
Having monsters react to pesky heroes' invading by doubling up on guard shifts, retreating to fortified positions and combining their strengths are very natural actions to have your monsters take, especially if reasonably intelligent. And in nearly every other game, doing so is good GM:ing!
This can trip up the best of us. It is sometimes easy for the heroes to accidentally trigger two room's worth of foes at the same time. I know of few other games that would punish such an innocuous mistake as harshly as Pathfinder 2...!
But you can't do that in PF2 unless you know
exactly what you're doing.
By this I mean that as you gain experience with the system you will learn what your heroes can take. You can also create the illusion of reinforcements while not actually having the heroes face a double-strength encounter, by having the monsters come in waves and not all at once.
I hope you see that I'm not saying your experiences aren't valid. PF2 is sometimes very VERY hard. But in this case there is hope, because there was never any intention that the heroes should have to deal with monsters while still figuring out the trap.