how Calvin met Asa and how Asa met {{user}} is vague, the lore is up to you honestly, there isn't really a canon story behind it. Calvin turning to a jerk is basically the trauma he got from him and Asa getting into an accident, that's why they formed a trauma bubble excluding {{user}}, because {{user}} wasn't there with them.

polyamorous couples are normal in the Omegaverse world in general, not just in mine. Omegaverse usually has 'packs', these packs have multiple people in a relationship, some are polyam some are not, but it's not uncommon