Um... I think Gosho won't make Conan and Shinichi seperate. There's already WAY too many characters in DC and adding in a new one who is an exact replica of the real Shinichi is a little too extreme for my taste.
And Ayumi is shown to be a major crybaby(sorry about that). She screams and cries and I can't deny that she's strong sometimes but I don't think that she'll be strong when her heart breaks. And Conan blushes and never tells her to stop as he couldn't tell her to stop. Firstly, she's her FRIEND and hence telling her to stop will be painful. And since she's surrounded by boys that are in love with her, she wouldn't know the meaning of rejection. And I feel that the character developments are not romantic developments. To me, they are just a deepening of friendship and trust.
And I can think up of another reason to why Conan don't want to reject Ayumi. Ai would personally kill him for hurting the girl who she treats as a younger sister. You can see the outcome.
And about him not growing up mentally, you might have a point there. I'm thinking of a new argument for that but I have to say that she won't still be on par with Conan. She's just going to be a normal teenager but she would never match Conan's intelligencs. Mitsuhiko maybe but certainly not Ayumi.
And I'll take note of the chocolate. White or Black?