Poseidon might be presented as a rather powerless god in the context of Percy being abused in fanfic. Despite being Percy's father, he could be constrained by the will of Zeus and other gods, which prevents him from acting decisively to save Percy. Hades, however, might be a more interesting character. He may have his own motives for helping Percy. He could see Percy as a useful tool against the other Olympians or simply as a victim who he can relate to, given his own outcast status among the gods. So, in fanfic, he might play a key role in Percy's journey to overcome the abuse.
If the grandfather is very warm and accepting, Shikaneji would feel relieved and happy. They might open up more about their relationship and their dreams. Nejire could start chatting excitedly about their future together, and Shinsou would relax and join in the conversation more. However, if the grandfather is not so friendly, they would be disappointed and perhaps a bit defensive.
Poseidon could play a regretful father. He may realize too late how much Percy has suffered and then try to make amends. For instance, he might secretly send gifts or powers to Percy to help him defend himself. Hades could be a secret mentor. He has a lot of knowledge about the dark side of the world and magic that could help Percy understand his situation better and find ways to fight back against his abusers.
Loki would probably be defiant at first. He has a strong sense of self - pride and would not easily submit to Thanos's abuse. He might hurl insults and threats at Thanos, even though he is in a vulnerable position.
James might be shocked at first. He could then feel a sense of protectiveness towards Severus. Maybe he starts to see Severus in a new light and tries to find ways to help him, like keeping an eye on those who abuse Severus.
The Autobots would likely be very protective. They are heroes after all, so they would immediately sense the vulnerability of the abused reader and do whatever it takes to keep them safe from harm. They might also be gentle and patient, understanding that the reader has been through a lot.