In a flashback/memory it is shown that the antagonist's mother was abused by the two men she was living with. The men were her father and brother, but she was an adult, so I'm calling it domestic violence instead of child abuse.
It's only one scene, and the protagonists talk about it afterwards. It shows physical and mental/emotional abuse.
Not successfully, but in 'Order of the Phoenix' a main character attempts to warn certain authority figures about the main antagonist.
A minor antagonist (who is an authority figure) insists that he's lying and forces the character to punish himself (involving repetition of a gaslighting phrase and scaring himself physically in a minor but visible way), and then insists that the character knows he deserves to be punished.
It's not gruesome, there's little to no blood, and the character has the emotional support of the rest of the cast to reaffirm what he knows to be true, but the scene exists.
Mrs. Norris is petrified, but she survives. The only other cats in the series are Crookshanks and arguably Professor McGonagall, a witch who can turn into a cat. Neither of them die.
Snakes are common both symbolically (snakes are the symbol of Slytherin, a student house within the wizarding school and its namesake, an ancient wizard who could speak to snakes) and as plot devices throughout the series. Aside from owls, which are common pets in the books that deliver mail, snakes are probably the most referenced type of animal over the course of the series.
No jokes that I remember but the love potion concept is rapey and this isn’t adequately acknowledged in-universe. In particular, Voldemort’s father leaving his wife for using a love potion on him seems to paint him as the one in the wrong.
Hermione has a jinx cast on her which causes her teeth to grow rapidly. When she has it reversed, she gets them shrunk to be slightly smaller than they were. There's also some discussion of her parents' work as dentists
The protagonist learns of a man who was killed years ago by a spell so destructive, all that was found was a finger.
SPOILER WARNING - SPOILER AHEAD (text reversed)
.htaed nwo sih ekaf ot ffo regnif sih tuc yllautca eH
the other comment is very random so clarification - it happenes in the chamber of secrets in chapter 7, i dont even know why "harry potter" exists on this site without referring to a specific book
the Happy Ending of the book has a main character achieving his dream of becoming what is basically a wizard cop, despite all his encounters with them in the series being them wrongfully imprisoning/assaulting his friends and family
She isn't trans, but as mentioned in "is someone misgendered," Rita Skeeter is referred to as "mannish." And JKR is infamously transphobic so good freaking luck trying to read the books knowing that.
In book 7, one of the henchpeople kills themselves and helps the protagonists escape. As a child I thought the henchman's magical body part did it without the henchperson's consent, but looking back as an adult, it was likely an impulsive suicide.
Nothing is explicitly said about Jewish people, but goblins (which are often considered to be antisemitic caricatures) are present and embody a lot of negative stereotypes.
No, but a minor antagonist, Rita Skeeter, is frequently referred to as "mannish". Also, the author is now notorious for her transphobic commentary on Twitter.
Yes, but only in the background cast. Many purebloods (like the Malfoys, Lestranges, etc.) are stated to marry their cousins a lot, to keep their blood "pure" (and probably keep their wealth in the family). But the main cast is free of incest, and no incestious relationships are put in the spotlight.