All I can say is: it would be unthinkable today---and
should have been unthinkable back then---not to conduct an autopsy on a deceased patient of Lanza's young age (let alone someone of his celebrity). I dearly wish that someone in the family had had the presence of mind to insist that an autopsy be performed. I'm also amazed that newspapers all dutifully reported that Lanza had died of a heart attack when there was no proof that he had.
The fact that Lanza died in a clinic during what was supposed to be a properly supervised period of rest and dieting also should have aroused some journalistic investigation. As it turned out, the only person who raised the alarm about possible medical negligence at the Valle Giulia was Dr. Frederic Fruhwein, but by then it was too late to conduct an autopsy (and the brave Dr. Fruhwein paid a high price for speaking out). Even so, Fruhwein's concerns should have been followed up, and one would think that journalists, at least, would have been interested in getting to the bottom of it all.
I know that Mario's parents were terribly distracted at the time---and of course Maria Cocozza did claim a decade later that medical malpractice had killed her son---but there really needed to be an investigation in 1959.
To me, Silvestri's refusal to speak on the topic indicates that, if nothing else, he was uncomfortable with the treatment that Lanza received at the Clinic. But if so, did he share his concerns with anyone? As Armando mentioned above, it would be nice to think that Silvestri left behind some sort of written account. A heck of a long shot, I know, but still....
I wonder if Silvestri ever read Armando's book? If he did, it must surely have unsettled his conscience.