How do enemies instantly realize that you're hostile after you've hotswapped into one of their friend's bodies?
Hotswitching into someone makes them throw their weapon, it probably causes them to cry out or give some other easily visible sign that they're being controlled.
It's also likely due to the fact that upon Hotswitching, your new body changes to Black, defining you as abnormal.
The double-barrel shotgun can sometimes fire three times before it's empty... How?
The gun also makes a cocking sound when picked up, in spite of it being double-barreled.
A possible Doylist explanation is that the shotgun's model was made on the assumption that it would fire two shots, but it was buffed mid-development to three. That's why the grenade launcher in Team Fortress 2 holds only four grenades despite having six barrels.
Everything in the game is represented in a generic style; there's probably actually several different styles of shotgun in the game, but the visual overlay you're seeing just represents them as generic double-barrelled weapons.
Some of the levels start you someplace calm where there's no action. Why does a guy in the bathroom suddenly try to kill you and why does everyone outside try to kill you as well? Surely they saw your character go INTO the bathroom, so why didn't they shoot you then?
It's possible they heard the commotion of you killing the first character, then went in to investigate. By the time you kill the second, they call security/the bartender uses the shotgun he has for self-defense.
