the reason becuase mickey is oblivious to xehanort in kh2 flashback and roxas resmpblance to ven is simple.its called plot manipulation.
u see mickey is a disney character.squareenix does not want him to have a direct impact on the story.like malifecint who was used mickey is a neutral good guy who indiractly affects the plot.the reason he doesnt recall these things is becuase it would add more layers to his character which would contadinct his status as a dinsey character.he is neutral good and helps the heroes without he himself doing much.after all he dint defeat any major villain.
u see mickey is a disney character.squareenix does not want him to have a direct impact on the story.like malifecint who was used mickey is a neutral good guy who indiractly affects the plot.the reason he doesnt recall these things is becuase it would add more layers to his character which would contadinct his status as a dinsey character.he is neutral good and helps the heroes without he himself doing much.after all he dint defeat any major villain.