In a Giyuu x Shinobu fanfic, the setting and plot can vary widely. Some might be set during the main events of the Demon Slayer universe, with them facing demons together. Others could be more of an alternate universe story, where they meet under different circumstances, like in a modern - day high school AU. The fanfic could also delve into their backstories and how those experiences shape their relationship. For instance, Shinobu's past with her sister could play a role in how she opens up to Giyuu, and vice versa for Giyuu's own traumas.
I would recommend 'Whispers of the Heart' for Giyuu x Shinobu fans. This fanfic focuses on their unspoken understanding. It's full of small moments that build up their relationship, like when Shinobu leaves a small flower on Giyuu's doorstep as a sign of her care. Also, 'Shadows and Sunlight' is worth checking out. It has a great plot that involves them having to go on a secret mission together and how that journey changes their relationship from colleagues to something more intimate.
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.