You wrote:
“Apparently the level of heroin was high enough that the M.E. says he could have been having convulsions.”
That is not supported by anything you just posted from the article...
What do you think a seizure is? That consists of convulsions.
“Police had said in a court filing that the second passenger reported hearing Gray banging and kicking through the metal divider before the fifth stop. Allan said that would not have been possible given Gray’s injuries, but he may have been suffering a seizure at the time, which could have caused the noise, she said.”
Allan is the M.E.