That's literally how police investigations work, you start assuming a crime has been committed and try to see if you can find evidence of a crime being committed.

You don't start assuming suicide by default. Not unless you're a corrupt cop anyway.

Yes you can. Mathematicians do it all the time.

But that's beside the point. I can prove you didn't kill yourself by showing that someone else did

This isn't proving a negative? Lol.

This whistleblower is dead, they would be suspect #1 along with close family and friends. This is literally what the first step of an investigation should look like.