Response: Good question: My answer is going to be based on federal and generic grand jury practice--the answer may be different in some states, since their rules vary from the federal/common law model.

The prosecutor should not be able to veto the grand jury's desire to call witnesses, because it is the GRAND JURY that investigates, not the prosecutor. I don't see any reason why the prosecutor cannot give his/her opinion as to the inadviability or lack of necessity for calling a witness, but if the grand jury persists, the prosecutor cannot veto their efforts. If the prosecutor thinks the grand jury has "run away" and is subpoenaing  people for improper reasons (harassment, curiousity, etc.), the prosecutor's logical recourse is to ask the judge who supervises the grand jury to dismiss them.

Response: Another good question, and again I'm drawing on federal and generic practice: This issue came up in the Oklahoma City bombing investigation when one grand juror wanted to subpoena witnesses and the prosecutor (and, apparently, other grand jurors) did not.

This gets into a grey area: Since it is the grand jury which is investigating (see answer to your first question) earlier email), the grand jury can call whomever it wishes, regardless of the prosecutor's wishes. But can one say that one grand juror is "the grand jury"? I think not--I think there has to be some minimum number of grand jurors (a majority?) who want to call a witness before the grand jury can/is required to do so.