It is legal to accuse someone of an astronomical tax debt and take their assets, I do not believe it would be considered legal for the IRS to make that accusation without papers showing the work they used to arrive at the accusation, and it would not be considered a valid chain of reasoning 'because the FBI asked us to'.

This, and other techniques you describe, have been used by the FBI in the past but as far as I know only against the relatively powerless. It seems silly to think the FBI would do it in this case.

If the FBI started doing that to employees or ex-employees I suspect Apple would leave the U.S.