A character is decapitated. Also, a major character dies of throat mutualization but this is not mentioned in the Published Silmarillion and only in a poem contained in HoME.
I can only guess that someone voted yes because a character remarried after their spouse dies but the original spouse is still conscious because of how the species' death works. The deceased character approves of the remarriage and there is no cheating.
The character Lomion (Maeglin) is implied to have been abused by his father. When Lomion is 80 years old, which is around the equivalent of a 16 or 17 year-old, his father attempts to murder him with a poisoned javelin, but Lomion's mother, Aredhel, takes the wound instead and saves Lomion's life.
It is said that Aredhel "was not wholly unwilling", but I do find that rather difficult to believe. She might have been under some sort of enchantments. It is unclear.
I don't think the character Lomien is talking about has very strong evidence for being autistic. I can see it, but not enough that I'd say its implied. There's another character who may or may not remind the reader of some traits of autistim (brusque, earnest, doesn't understand figures of speech or sarcasm, seen as weirdly mature as a child but infantilized a bit as an adult, doesn't play with childhood best friend but instead likes to just be near them, nonverbal for a while at one point from trauma) but that's still a theory and they are not abused BECAUSE of it. They are described as "less loved" than a sibling for these traits in childhood but they are still very loved.
Sort of. The Silmarillion is an alternate history, so you are canon to it and are the conceit is that you are reading a translation into English of old texts. Tolkien created several frame stories.
The words "mad" and "lame" are used if that counts and in ch21 a couple characters act ableist towards some others (but aren't portrayed as right for it) and one of these characters was having a mental breakdown.
One main character is sexually objectified by the main villain in ch19. It's mentioned briefly in the published Silmarillion, but it's more explicit in the Lay version and the language may be triggering.