How to answer part A of the OP? Should I answer as an umpire, or as a former assistant SID who also did the baseball public address announcing?
What the hell, I'll go with the latter:
If it is the visitors who are sending the wrong batter up, you announce the wrong batter's name loudly and drawn out (so your team's head coach starts thinking to himself: WTF?).
If it is your team who is sending up the wrong batter, you announce the proper batter's name so that the wrong batter will stop dead in his tracks.
Seriously (as this is an umpire board) I have no changes to what was posted above.