I don't know why MacNeill was in such a hurry for the autopsy to take place but it would not be because the drugs would show up any more as time passed. Metabolization stops at time of death. Period.
Theoretically there could be some degradation of certain substances within the blood after death - but not metabolites of drugs to my knowledge. What is there is there.
For instance, in the case of the limo driver in Princess Di's death blood samples were taken on the day the car wreck happened. When levels came back so high on certain items (alcohol and carbon monoxide) further samples were taken some 3 days later. Those measurements were almost identical to the originals.
Not that in the limo driver's case it wasn't alleged that some other person's blood was substitued. That allegation stems from the very high levels of carbon monoxide that have never been explained.