The FBI will not be given such a key, so what John McAfee says about it is irrelevant.
Apple can maintain full custody of the phone and the software until the data is extracted from the phone, after which time Apple can restore the original operating system before handing it back.
Imagine yourself going into court to present evidence taken from a phone, and explaining that you had allowed a third party to temporarily install software onto the phone without giving you a copy to examine.
Next, imagine yourself on TV asking the viewer if they've been injured in an accident, as that would be the direction your career is likely to take after such a fiasco.