A couple of things. First, there's no concrete evidence such a document exists. Second, if it did, it doesn't change the authenticity of the Gospel of Luke, any more than having a draft of the Declaration of Independence changes the authenticy of the Declaration.
However, if such a document existed, it would be of significant argheological and historical significance.
Not necessarily, since only one writing, the Gospel of Luke, was written with the intent of being shared. We shouldn't presume that everything Luke (or any other scriptural author) wrote is on a par with what is in scripture. In the same manner that we shouldn't presume that everythign Thomas Jefferson wrote is on a part with the Declaration of Independence.