<< As soon as you die and decomposition sets in, one byproduct is alcohol. So I wonder if it's even possible to determine whether alcohol in a body has come from drinking or decomposition. >>
Apparently it is possible.
On December 17, 1977, United Airlines cargo flight 2860 (DC-8F) flew from San Francisco, California to Salt Lake City, Utah (KSLC). Approaching KSLC they experienced electrical problems, were given holding instructions at their request and crashed into some local area mountains. It took some time (several days?) before rescuers reached the site.
Alcohol was found in the S/O's (flight engineer) remains and it looked rather bad. Some time later it was determined that the alcohol was from decomposition.