It is still two different issues. Missing and missing in action versus accounting for total losses. One is made up of a great long lists which one usually grabs from the DPAA accounting, and which is, in reality, not listings of missing or missing in action, but, rather, listings of unrecovered bodies . . . there is a difference. One might note that the DPAA site specifically uses the phrase “service personnel not recovered following WWII” when it presents its listings. It does head those lists as “missing in action”. The second issue, which addresses the questions posed by the OP, is how the services accounted for the total deaths of the conflict, in which the missing and missing in action are counted and included.
The original questions were: “Does the 400,000 deaths number include the 80,000 MIA number? Or could you technically say that 480,000 Americans were killed or missing in WW2?”
Regardless how one, today, might want to count the US missing and missing in action from World War II, the answers are quite clear:
“Does the 400,000 deaths number include the 80,000 MIA number?”
Yes, the missing and missing in action are, indeed, by law, which is clear and unambiguous, included, counted, in the total deaths count.
“Or could you technically say that 480,000 Americans were killed or missing in WW2?”
No, you cannot even “technically” add the missing and missing in action to the total deaths as that would be counting them twice. Well, s’truth, you can if you want to but you would come up with an erroneous total which I am sure someone would be happy to challenge.
US services accounting for the missing or missing in action were covered under Public Law 490 of the 77th Congress which I mentioned in my earlier answer. The US Army casualty report extracts in that answer briefly describe how the Army defined and applied the law. If you wish, you can find the referenced Public Law 490 in its entirety here: https://www.loc.gov/law/help/statutes-at-large/77th-congress/session-2/c77s2ch166.pdf. The various sections of this law cover the potential eventualities and the process to be followed; of specific note is section 9, where it says:
“Within the scope of the authority granted by this Act, the determination by the head of the department concerned, or by such person as he may designate, of the status of a person in the military or naval forces, the Coast Guard, the Coast and Geodetic Survey, the Public Health Service, or civilian officers or employees as defined in paragraph (a) (3) of section 1 of this Act, or his direction relative to continuance, temporary suspension, or resumption of payment of pay and allowances, or finding of death, shall be conclusive.”
“ . . . or finding of death shall be conclusive . . .” sounds definitive. That means an individual is no longer listed as missing or missing in action, it means the individual is listed as one of the categories of war dead.
It is all very straight forward. For example, US Army combat deaths:
Total US Army deaths among battle casualties, all theaters and branches including USAAF, as of 31 December 1946 were 234,874.
This breaks down as
Killed in action = 189,696
Died of wounds or injuries = 26,225
Evacuated to US, died of wounds or injuries = 84
Captured & interned total = 124,079
Captured & interned, returned to military control = 111,426
Captured & interned, killed in action = 3,102
Captured & interned, died of wounds and injuries = 453
Captured & interned, died of other causes = 9,098
Missing in action total = 30,314
Missing in action, returned to duty = 24,098
Missing in action, declared dead = 6,058
Missing in action, died of other causes = 158
Add up the deaths in these three categories and you get, hmmm, 234,874.
For non-battle deaths there were a total of 92,656
Non-battle accident aircraft = 27,628
Non-battle accident, non-aircraft = 29,224
Non-battle disease = 26,518
Non-battle other = 9,286
There is absolutely no doubt in my mind that the bodies of a not inconsiderable percentage of those noted as killed in aircraft accidents were not recovered, thus making them “missing” (“missing,” not “missing in action”) and probably a percentage of bodies from the accident non-aircraft deaths were likewise not recovered, putting them also in the “missing” category, but for death casualty accounting purposes, they are dead and are recorded as same.
Add up all these non-battle deaths and apply that sum to the battle deaths and we arrive at 318,274.
Sniff around the internet and one can find the total US Army casualties, here’s a quickie:
https://www.nationalww2museum.org/students-teachers/student-resources/research-starters/research-starters-us-military-numbers and what do we find? Hmmm 318,274 . . . looks right to me.
At that same site we see for the US Navy, US Marine Corps, and US Coast Guard deaths 62,614 and 24,511 and 1,917 respectively. There are available on micro-fiche lists of USN killed, originally produced in the Navy and Marine Corps Awards Manual, but it takes some sleuthing about to use the data in a meaningful manner. And, unfortunately, it does not include all US Navy losses; others may be found in files at NARA covering miscellaneous losses such as naval gun crews assigned to merchant ships and US Army vessels. Working to determine a formal date of death for the individual missing requires some careful researching. Sometimes it is tedious but can be done. We can, however, work backwards through the gross numbers to satisfy ourselves that the missing and missing in action are included in these total numbers.
Looking at https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/u/us-navy-personnel-in-world-war-ii-service-and-casualty-statistics.html
We find the US Navy reports total combat deaths due to enemy (a general category) all officers, enlisted, and officer candidates, as 36,950, about 59% of its total reported deaths.
Killed in action = 30,831
Killed in action, air combat = 3,173
Died of wounds = 1,837
Died POW = 919
Died other deaths due to enemy action = 190
Next, we can find that the total of deaths reported due to other than enemy action was 25,664.
Natural causes = 5,533
Aviation accidents = 8,184
Other deaths, 11,947
Let us see . . . 36,960 + 25,664 = 62,614 . . . check.
For a couple of micro looks, one might consider the loss of submarines over the course of the war. The USN reported 52 submarines as known lost or lost as overdue from patrol. Lost with these were 3,506 officers and men. Their remains were, for the most part, not recovered and never will be. These individuals appear in the DPAA listings, but when one double checks, one finds that they were declared dead as required by law and are included in the deaths accounting.
For example, plucking a name from those known losses and trying to keep it simple with one of the more well-known such, CDR Samuel D Dealey was commander of USS Harder, lost on 24 August 1944. Dealey appears on the DPAA rolls as unrecovered; he was reported as missing in action. A year later, 25 August 1945, his widow, Edwina, was presented with his posthumous Medal of Honor. We can also find his name on the list of those killed aboard Harder in the US Navy’s “United States Submarine Losses World War II” (a nice volume if you can get your hands on one, but the USN Heritage & History site has it available in an on-line format at https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/u/united-states-submarine-losses.html). The bodies of he and his crew were never recovered (in Navy vernacular ‘BNR’ – Body Not Recovered) and are, therefore, counted among the US Navy’s war dead. They, the crew of USS Harder, were all officially declared dead on 2 October 1945 and are memorialized at the Manila American Cemetery, see https://abmc.gov/print/certificate/462598
Another easily ferreted example would be from those killed aboard USS Arizona at Pearl Harbor. The accounting of how many, and who, were lost came not from counting bodies, but from comparing muster rolls to survivors. Thus, and without looking very hard at all, on the first line of the DPAA Navy unrecovered listing is Aaron, Hubert, F2c, lost on 7 Dec 1941 on USS Arizona; his body was not recovered. If you were to check the US Navy’s published state by state reporting of casualties, look in Aaron’s home state of Arkansas and, lo, there he is, the very first entry under “Killed in Action, Died of Wounds, or Lost Lives as a Result of Operational Movements in War Zones.” State by state loss registers are available from the NARA website.
So, F2c Hubert Aaron, USN, who, as an AS, was received for duty aboard USS Arizona on 1 January 1941, was lost in the attack on Pearl Harbor. He is listed as missing in action on a 31 December 1941 USS Arizona muster roll and currently still listed as remains unrecovered. He was, however, declared dead by the Navy as of 7 December 1941 and is counted as killed in action. He is memorialized at the Honolulu Memorial, see https://www.abmc.gov/print/certificate/476944.
Most of this is a very simple exercise with the knowledge of the data underpinning the numbers. I will not go through the exercise for the US Marines or US Coast Guard, anyone wanting to chase their numbers is invited to do so.
At the risk of being repetitious, the missing and missing in action from US service in WW2 are not carried separately; they were, at some point, declared to be dead, as required by law, and were added to the count of dead for the particular service. With their numbers then included in the total deaths for each service, and it would be a mistake to count those whose bodies were never recovered as being in addition to those counted as service deaths.