if a survivor goes and sits in a chair without you making him do it, don't bother him, he'll be reading a book (the more you have, the better) and it will help reduce sadness. an armchair would obviously be more comfortable, therefore helping more. unfortunately, there isn't any "guide" that tells us exactly what to do and how things will work out according ...
You do miss these events when you skip the day.
i can not provide a Source for my observations but with around 15hrs into the game I feel confident to give you this advice:
The trader usually shows up quite early. The latest guy i had was around 2.pm but this seems to be only the first one on Day 2/3. Normally he is there within 2-3hrs into the day.
The moment you go scavenging, it counts as "stayed awake that night". Even if you just pick up a pile you left the night before, and are back before 10 PM. So you cannot sleep or guard that night.
Yes, they will be hostile againts the chartcher that made them hostile.
But the madness only goes to him, meanning, If you try and go with a different person, They'll go back to netural on you.
They won't cause any harm to Katia.
You can only use an itens when the game "enable" its use for you
You can use Meds when a character is ill,
And the books, its good to keep some of them, but to burn it you should convert it to fuel in the workshop.
There are these wiki with tips and survival guides
From what I experienced, if your safehouse contains a tree-house in the backyard, then garage will not show up in that story playthrough. I still need to verify this with more playthrough. If someone can positively verify, please edit the answer.
I've completed multiple (10+) play-thrus and every NPC stays dead or gone, the bodies of your PC's that die will stay until looted as well.
So you after clearing you can safely send unarmed people in to pick the goodies.
I've not played enough to know if any locations have additional mini-story-arcs that may bring more NPC's to the location at a later time ...