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.
Some friends were likely horrified. The Chris Watts case was so heinous. His actions of murdering his family were unimaginable. Friends might have initially been in disbelief, thinking that the person they knew couldn't be capable of such a thing. They could have felt a sense of betrayal as well, since they thought they knew him but he had this dark side.
One common theme is the exploration of their relationship. Given their different personalities in the original work, fanfictions often focus on how they bridge the gap between them, like how Giyuu's stoicism and Shinobu's more cheerful yet calculating nature interact.
Draco's Slytherin friends would be quite shocked. Some might shun him, thinking he's betraying the Slytherin ideals. However, a few might see the advantage of having a connection to Harry Potter. For example, if there are any threats to Slytherin as a whole, Harry's influence could be useful. So there would be a split in how Draco's Slytherin friends react.