>>281568882
Haibara was intended to be introduced far earlier in the narrative, and the Detective Boys actually only exist because of her, rather than her being an addendum to the group. They were to act as a 'cover' for her character to blend in, whereas Shinichi doesn't need that given his closer relationship with Ran and Kogoro, plus the overall arc of the Detective Boys 'rehabilitating' Haibara and giving her a chance to live out her childhood.

As for love rivalries, I imagine this is an endgame scenario thing. The final arc of Conan, rather than a grand showdown, will probably be something following said grand showdown- that being, should Shinichi return to being Shinichi or stay as Conan. Yaiba had a similar lowkey affair for its final arc, and there is a lot going on to suggest that the change back will be something dramatic. Namely the fact that all of these relationships within the show exist because of Conan rather than Shinichi, and that Conan leaving would be overall more devastating for the cast than Shinichi leaving, which would be primarily upsetting for Ran. Haibara's very obvious love for Shinichi plays a role in that.