Sommie is noted as having belonged to previous divine dragons on the Somniel. Sommie is immortal and is mentioned as "being sad everyone leaves" in its journal entry.
I'm gonna mark this as yes just to be safe, as two characters are referred to as "defects" by their parents, although not for reasons related to any real world disabilities the same logic is being applied.
Many characters do suffer from PTSD, the most prominent being the Brodian royals, the Elusian royals, Sapphir, Vander, and Mauvier. Most of it is in supports exclusively but there is an unskippable boss regarding a characters trauma response
In two characters supports there is a gag regarding a laxative being mistaken as a legendary potion. The support is between Amber and a spoiler character, and is their B support
A late game chapter has a village attacked by the antagonists and razed to the ground. If you talk to one of the villagers, he'll tell you that he's happy to have survived but he no longer has a home to go to.
I have not seen any spiders in the game. However, do note that one of the Emblem characters, Eirika, comes from a game where giant monstrous spiders are a common enemy type.
The main villain (noticing a pattern?) is prone to abandoning or outright killing his children. In one instance, the abandoned child gets away and is adopted by someone who treats them with kindness.
There are scenes revolving around a group of villains that refer to themselves as family where they will be abusive and violent to those under them in the family dynamic. And the main villain is cruel to his children physically and mentally.
A sympathetic/heroic character is occasionally put under evil possession and set out to do horrible things. When they are their normal selves, they have no recollection of doing this. This leads to a lot of gaslighting, both intentional and accidental, over the course of the game. It does work out well for them in the end however.
The main villain is explicitly stated to have killed several of his own children. Furthermore, some of the playable characters are young children and can die in battle.
The Emblems could be thought of as ghosts, as they are apparitions of long deceased heroes from other worlds. They are not at all played for horror however.
A major character is placed under demonic possession and forced to do terrible things that they are not conscious of and have no memory of when they awaken. Several deceased characters are also brought back from the dead as possessed soldiers.
One of the mechanics of the game sees the protagonist awoken by one of their allies. If you wear headphones, the game uses a binaural audio effect while this happens. Some of the sound effects used can be unpleasant for players with misophonia concerns. However, you can skip the sequence while still getting benefits from them.
While character ages are not displayed in game, the protagonist is a dragon with a lifespan beyond human years, so by that fact alone anyone they romance is going to be significantly younger than them. That being said, these are optional and the majority of supports end platonically.
There's a weird alternate universe version in the DLC, where the child of one reality's Sombron can fall in love with the child of another reality's Sombron, making the two kind of but also kind of not siblings.
One of the playable characters is a large, muscular man who is prone to crying when overwhelmed with emotion. This is sometimes played for laughs, but usually not in a cruel manner.