Tom from your list above here is the Shiloh KIA men as listed in the roster of the 14th Iowa, minus ones we have already posted about:

No William Creedon is found in the 14th Iowa roster nor in the Sailor Soldier System

Eagles, Elias. Age 40. Residence Mount Pleasant, nativity England. Enlisted Oct. 25, 1861. Mustered Nov. 5, 1861. Killed in action April 6, 1862, Shiloh, Tenn. (Company I)

Gravett, Thomas H. Age 24. Residence Vandalia, nativity Pennsylvania. Enlisted Oct. 4, 1861. Mustered Nov. 1, 1861. Killed in action April 6, 1862, Shiloh, Tenn. (Company E)

Lane, Samuel. Age 19. Residence Hillsboro, nativity Iowa. Enlisted Oct. 14, 1861. Mustered Nov. 2, 1861. Killed in action April 6, 1862, Shiloh, Tenn. (Company F)

Lockman, Samuel B. Age 42. Residence Bloomfield, nativity Kentucky. Enlisted Oct. 1, 1861. Mustered Nov. 5, 1861. Killed in action April 6, 1862, Shiloh, Tenn. (Company I)

Zike, Francis M. Age 20. Residence Shueyville, nativity Kentucky. Enlisted Oct. 2, 1861. Mustered Nov. 2, 1861. Killed in action April 6, 1862, Shiloh, Tenn. (Company F)

Tom the major flaw in the "Isaac is really Zach" theory still is that we have no real clue that Zach died any place close to Shiloh, or any time soon after his discharge. If as a Park Ranger you have easy access to NARA records, it might be beneficial to look into their records for Zachariah Walker. I'll keep looking for anything that might rule him in or out as a valid candidate for the man mistakenly identified as "Isaac Walker" said by the ROH to be buried at Shiloh. So far I still think it most likely must have been Jenks' body who was wrongly identified as Walker, but I'll keep trying to rule him out if possible too.

PS Gravett served from the same hometown and company as your Zach Walker, they enlisted a few days apart, probably were messmates.