Howard Hughes dead before leaving Mexico?

One of the most controversial statements in The Final Days of Howard Hughes is the eyewitness report that Howard was wheeled out of his top floor penthouse suite at the Princess in Acapulco with a sheet over his body.

Was Howard dead before he left the tarmac in Acapulco? What would this mean to the story, where an official autopsy was never performed and probably should have been in Mexico by Mexican authorities before he was allowed to leave.

Also, why, if Howard was in the midst of a medical emergency that required immediate attention, why was he allowed to remain inside a hot plane which stayed on the tarmac for three hours before departure for, not the closest hospital in Mexico City, but to a far off hospital in Texas?