Josy knows about it only if your MC decided to tell and share with her. Scene you mentioned is canon and plays similar dependless on those choice. Same goes for Sage. She is either know or not - depends on your choice. Sarah at the party too and she is also aware. And you'd better focus on Sarah here. Why the hell Melanie should rat out Chad now and to Caleb especially if she didn't do it for years, keeping their secret? Can you follow at least a bit of the logic?
Josy doesn't have such intentions and even not sure to tell Sage or talk about it with her. Why the hell you think JOSY would talk about it with CALEB? Sage also saved Chad's secret. And again - both Josy/Sage or both know about it only if you told them in the past. This ruines all of your assumptions and theory again.

So... Josy/Sage/Melanie - they are all Magnar minions and they do what he asks for and want?

Again, how you will deal with fact that their knowledge in optional and depends on our choices? And that Melanie would never rat out Chad & Troy?