A husband psychologically dominates his wife, but there's nothing physical (at least not shown or talked about). The wife is definitely afraid of her husband's anger, and suppresses parts of herself because of him.
Not for the usual reasons, in the clinical sense, but yes, someone does harm themselves quite badly, in a way that resembles what many people do to handle their emotions.
Perhaps not in the strictest sense, in that no one is led to believe that they're actually mentally ill, but two people completely ruin two other people's trust regarding whether what's going on at any given time is for real or acted. The end goal for that behaviour doesn't seem to be to ruin their trust, but it definitely has that effect, and in any case it's a case of complete disregard for other people's feelings and mental health. I imagine that people who've been subjected to gaslighting should stay away from this movie.
No, but there's a scene in which a group of people shot with potato guns at an old car. Seeing a car being damaged, e.g. a windshield cracking, may be a trigger for some, I imagine.
Yes and no. There's a scene in which someone is "shot" with a prop gun and pretends to die. Even though we know in advance that it's an act, it looks completely real, so it's probably a trigger. Also, someone is hit on the side of his head with a potato from a potato gun, but it's an accident.
A woman's drinking problem is referred to numerous times, for instance with the line "Have you started drinking again?" (which appears to be the case).
Answering that would be too much of a spoiler. I can say, however, that there are no death scenes except for a fake one, in which the mother pretends to die as performance art (we know beforehand that it's an act). --- In order to leave a comment, I had to choose either "Yes" or "No", which is a bit annoying in this case... I chose "Yes", but you shouldn't take that as an answer to the movie's big mystery. I chose "Yes" because of the faked death scene as well as the fact that two grown kids find themselves in a situation where concrete evidence points to their parents being dead.
Someone is injured and is given, among others, Percocet at the hospital. He continues to take them after he's been released from the hospital. Judging by several lines, he most likely has a problem with drugs or has had in the past.