Denying a defendant access to the testimony of a witness is, of course, contrary to the 6th Amendment.
Oh, I meant to remark in my previous, but forgot. On skimming through an exchange between Nelson and West, I notice Nelson being very focused on the question of "relevancy." Crump is not a witness to the event, and his take of Witness 8's remarks is "the jury's job," not his.
Anyway, not to argue the point, just to say that Nelson was focused on the relevance of Crump's observations, to the question of whether or not Zimmerman's use of force was justified, vs. Zimmerman being guilty of murder. Generally, there are many other angles of relevance. Relevance isn't a matter that pertains only to the ultimate question, it can also pertain to any number of included sub-issues. I don't think West "connected" with Nelson's thought process, to make that simple point, and she is apt to think/believe that Crump simply doesn't possess any information that is relevant.
THis is sort of related to your observation about the 6th amendment and witnesses. What is Crump a witness to? Not to say he isn't a relevant witness - he is, I think, to the credibility of Witness 8 - just to say that another way to view the thought I'm trying to get out is that Nelson may view Crump as "not a witness" because he has no first hand knowledge of the incident, or of Martin, or of Zimmerman.