Mercer's letters home to his family are available in the Southern Historical Collection at UNC in the Mercer Family Papers. Here is some info on the capture of the 20th NC flag:
Several soldiers from the 97th New York also managed to wrestle away the battle flag from the men in the 20th North Carolina. “Company C was the first at the ditch,” Lt. Ebenezer B. Harrington from that regiment remarked. “Sergeant Sylvester Riley was the first to grab the Twentieth North Carolina colors and immediately handed them over to me.” Lieutenant Harrington noted that he “turned them over to Colonel Wheelock as soon as we had conducted the prisoners...from the meadow to our former position whence they were sent to the rear.”
Soon afterwards, Colonel Wheelock was ordered to turn over the captured flag to General Baxter. “But, the colonel would not comply, saying ‘my regiment captured these colors and will keep them,’” Lieutenant Grant from the 88th Pennsylvania reported. General Baxter immediately placed him under arrest. Wheelock responded by running his sword through the flag, tearing it from the staff. He then began waving “the torn banner’ at the enemy in a “taunting manner.” When an officer from a nearby company joined Wheelock and began waving the staff, he “received a ball to the forehead and fell dead.”
Sources: Ebenezer B. Harrington to Isaac Hall, April 29, 1890 in Hall, Ninety-Seventh New York, p. 141, Grant, “The First Army Corps,” in Brady and Freeland, Gettysburg Papers, 1:263.