No, I doubt Mickey would be playable in Birth By Sleep, or at least by your own volition. If he makes any playable appearance whatsoever, it'll most likely be like in Kingdom Hearts 2, where Sora passes out, and Mickey comes onto the scene during certain battles and with the proper post-death option selected.
And I think Master Xehanort's Apprentice, also known as the Darkness Soldier, is a so-far-unknown character, but I believe he was possibly friends with Terra, Aqua and Ven earlier on, only to betray them, or be controlled by Master Xehanort to betray them. Ven rushing towards him in the Birth By Sleep trailer? Perhaps he's trying to run towards him and change his mind, in the vain hopes that he is still in control of his own mind, and not under the total control of Master Xehanort. Terra then stopping Ven and rushing in himself? Terra is less forgiving, putting his hand on Ven's shoulder to stop him and to say something like "It's no use", and then rushing in himself, never at any one point being remorseful, only seeing a deceit and betrayal of a former friend who broke their trust.