[thaumasiotes]

> But postmortem is literally “post death” in Latin.

Sure, that's true. It's not really relevant to people who don't speak Latin.

> It’s synonymous with autopsy!

This isn't true. I have no idea how you got here. In English an autopsy is a surgical procedure performed on a corpse for the purpose of identifying the cause of death. In particular, it's a noun. "Post-mortem" in the etymologically literal "after death" sense is an adverb or adjective identifying whatever it is as taking place chronologically after some contextually-specified death.

But, you seem to put some weight on the etymology of a word independently of its current meaning. In that case autopsy "literally means" to witness something personally, as opposed to hearing about it from someone else. (It's "self-eye" in Greek just as post mortem is "after death" in Latin.) Somehow I doubt that's what you had in mind...?

[brabel]

An autopsy is an exam that is performed postmortem. And that’s really how people are using postmortem as a noun for the analysis that they perform after the “death” of a company, so it is a synonym at least approximately, if you don’t see that I guess you don’t have a very good imagination.

[thaumasiotes]

Try and find me some sentences out there on the internet in which one of the words could be reasonably substituted for the other one.

They don't mean similar things and they aren't used similarly. A postmortem is, as you note in part, a report or a meeting for the purpose of producing or discussing such a report; an autopsy is a surgical procedure. They're as "synonymous" as the words "cigarette" and "cancer".

[brabel]

> They're as "synonymous" as the words "cigarette" and "cancer".

Now you get it.