You only roll a single Stealth check when you start an encounter by Avoid Notice.
Unless the GM has other ideas, of course (the rule says "usually").
My PCs all Avoid Notice and then say: I want to roll Perception instead because it’s better, and I respond: then you’re not going to be very hidden, are you?
In most cases, there are no bonuses to be gained by either going first or attacking from stealth. Only characters and monsters with specific abilities gain anything. There are zero general rules of the "you gain advantage when attacking from stealth" sort, or "you're flat-footed until you have taken your first action". If you can't point to an ability specifically granting you some sort of bonus, there isn't one to be had.
It’s worded awkwardly, but if you are
hidden, the target is flat-footed to you.
I'm getting the definite impression the idea is that you go into encounter mode when every creature is on the battle board, and combat can ensue right away.
I don’t think
fighting necessarily starts, but that sounds about right. If the party is trying to stage an ambush, you wouldn’t start the fight when the ambush sprung by when the enemy comes into the scene. You’d have them do their thing unaware of the ambush, and then the PCs would spring the ambush on the designated signal.
And yes, this is in sharp contrast to other D&D games where it feels natural to do things turn-by-turn as soon as that feels right. Here the transition from Exploration to Encounters feel wonky if you're dropped into turn-by-turn mode before the fight has started; i.e. rolling Initiative with no fight.
Yep. I think in most other games you’d e.g., start when the above ambush sprung. On one hand, that makes sense given the absence of a discrete surprise round. On the other, it potentially tips the PCs off that an encounter is coming even though nothing is on screen yet. You could do secret Initiative checks in that situation, but that’s definitely not the norm in PF2.
I guess you could have them roll initiative at random times just to keep them on their toes. “So you go to use the chamber pot, roll initiative.” Have them believing all the chamber pots are mimics in short order.
Of course you can run the game otherwise, but official adventures clearly weigh towards the "and the monsters are right in your face - roll initiative!" where the idea that you would want to avoid just that and keep the beasties at a distance as long as possible seems to just not occurred to anyone involved.
If the PCs know or suspect there are monsters in the next room, they can Avoid Notice as they approach. When the encounter starts, everyone Delays but the slowest one in the initiative order, who kicks in the door. The rest of the party returns to the initiative order at the end of the kicker’s turn, and they rush inside to beat up the monsters for their treasure.
I swear I can remember having arguments over whether you got surprise when kicking in the door. Did Paizo accidentally (almost*) fix it?
* The asterisk is because you can only return to the initiative order at the end of someone’s turn, so you can’t guarantee a bad enough result that you go after all the monsters (and consequently get a ‘free’ turn beating them up). If they survive, they get to return the favor, but the party should be able to drop a creature or two and tilt things in their favor. Also, extensive footnotes are bad for jokes.