You know I am curious how you will handle the establishing of the poly. Aside from the obvious bonding with the Sanses to a point you are comfortable dating them but also just thinking about Readers home AU. Can someone have poly soulmates? Because it wouldn't surprise me if their AU had some anti poly opinions considering there's so much focus on having that special 1 person who completes you. Horror implied it's more normal in other AUs since his proposal's issue was not asking Dust who was part of the relationship first but I imagine Reader has some internalised beliefs to tackle. I look forward to seeing how everything develops.
There are some very anti-poly subtexts in Reader's society, since someone can only have one soulmate. Reader handled the general concept really well when Horror proposed, but she was also talking to someone who had tried very much to end her life. No telling what feelings will crop up when she inches closer the actual reality of the situation.
And by 'no telling' I mean I'm not telling you, specifically. But I think you'll either love or hate how it goes down!
The assumption that it's more normal in other AUs is also correct, particularly for Horror's AU. He comes from specifically from a baraverse where there's a bit more of a 'pack' mentality, everyone is a little closer to their instincts. Even more so when food started running low. For him, it's only logical to form a bond with Reader to boost her protection, she simply needs more people. And he's already started to see what Dust sees in her.