Navy, Marine and Coast Guard casualties for Missouri can be found here, unfortunately, there is no county list:
The Army list of WW II Casualties is organized by county:
Dear Ms. O’Brien-Blair,
Thank you for posting your request on History Hub!
The principal World War II Army casualty list, World War II Honor List of Dead and Missing (Washington: War Department, 1946) is arranged alphabetically by state and thereunder by county. Each state booklet includes a foreword describing how the list was compiled, a statistical tabulation by counties and by types of casualties. Each entry includes decedent's name, serial number, rank, and type of casualty. The towns within the counties and the dates and places where the casualties occurred are not mentioned. World War II Army casualty lists for the state of Missouri are available online through the National Archives Website at https://www.archives.gov/research/military/ww2/army-casualties/missouri.html. Page 26 covers Pettis, Phelps, Pike and Platte counties.
The combined list of Navy, Marine Corps and Coast Guard casualties in World War II, State Summary of War Casualties (Washington: Navy Department, 1946) consists of separate booklets for individual states and lists those men on active duty whose deaths were caused by enemy action or from operational activities against the enemy in war zones from December 7, 1941, to the end of the war. Casualties which occurred in the United States, or as a result of disease, homicide, or suicide anywhere are not included. Entries in the list include the name and rank of the decedent, and the name, address and relationship of his next-of-kin. World War II casualty lists for the state of Missouri are available online through the National Archives website at https://www.archives.gov/research/military/ww2/navy-casualties/missouri.html. This listing is arranged by type of casualty and thereunder alphabetically by last name of the casualty. Unlike the Army list, it is not subdivided by county.
We hope this information is helpful. Best of luck with your family research!