I can't prove anything either, but the OD theory sounds more plausible to me than the heart attack story: he was only 27. But that's only a guess on my part, based on nothing.
Too many drugs ... Heart stops ... heart failure ... heart attack. It's all the same, whether from cholesterol-clogged arteries at 65 years old, or coronary artery spasm or narcotic myocardial suppression at 27. Drugs can kill, and they often do.
John Belushi, John Entwhistle, Brian Jones, Keith Moon (yes, alcohol is a drug!), Janis Joplin, Mama Cass, Jim Morrison, Jimi Hendrix, Keith Richard*, the list goes on. How much more great music would there have been ...
* Yes, folks, it's well known among the cognoscenti, he actually died long ago. His appearances now are entirely faked, like a high tech "Weekend at Bernie's" sort of thing.