1 They're creepy low CR monsters that can appear just about anywhere?
2 Do you mean lack of darkvision? It won't help them, but they have characters who can do Light/Daylight spells so they'll probably be OK.
3 Sounds like a good idea to me.
1; A creepy creature that represents a different type of fight to the PCs - as opposed to something like zombies and skeletons which basically just walk forward and try to hurt you, stirges are basically big flying mosquitoes that latch on and deal CON damage, something that can be very inconvenient to PCs, especially in the middle of a dungeon dive. As well, they can be difficult to fight because of their flight. But, they don't necessarily represent a lethal threat to PCs despite their powerful abilities, since they leave after stealing 4 points of CON. A good way to freak them out without killing them and to have a different style of low-level combat.
2; Not at all! In fact, a lot of people tend to ignore the lighting conditions, but forcing them to bring torches or light spells and playing up the creeping shadows and the things moving just beyond the light's reach can really set a good atmosphere. The first time they're ambushed in the dark by scorpions, they'll be sure to bring a torch in the future.
3; I personally did away with trust points because it seemed overly mechanical and gamey towards the atmosphere (and being able to adjust the town's attitude by ear is more in my wheelhouse), but playing them out in the open can help the players feel a bit more invested in the town and the people there if they are the smash and grab type of players.
The Paladin (and to a lesser extent, the cleric) is going to clean up this AP - seriously, immunity to fear and smite evil on undead and evil outsiders becomes absolutely ridiculous as the campaign wears on, but overall the group will likely do well in this AP.