If this is true then why would Yen Sid not think it was important. If he did tell wouldn't that mean that Sora would go tell Mickey about Roxas and how he look EXACTLY like Ventus. They would then know where Ventus's heart is and all they would have to do is go find Terra's. Seriously I feel like this would have made things a lot easier.
What would Yen Sid not think being important? Telling Sora about Ventus and the others?
As already said, until Re: Coded when Mickey discovered the hidden memories and Roxas' appearance, there was no knowledge for
both Yen Sid and Mickey that the BBS crew is in any way connected with Sora.
Yen Sid not telling Sora anything before the end of Re: Coded is the same reason as Mickey's: The problems surrounding the BBS trio are
none of Sora's business in his eyes so he sees no
need to say anything.
Again, Re: Coded is the
first time they learn there even IS a connection.
Sora also cannot go and tell anyone about Roxas looking "exactly like Ventus" because he doesn't even know what Ventus looks like. Sora never met Ventus in person so far, they only had contact via their close heart connection and in both cases Sora was still very little (a newborn heart and a 4year-old, respectively), so he obviously doesn't remember these events anymore.
Furthermore, why would he suddenly feel the need to go up to Mickey and start talking about his Nobody when he doesn't even speak about Roxas with Riku and/or Kairi, who are arguably closer to him than Mickey?
There is a difference between knowledge we as the audience have and the one individual characters have.