> What would be the precedent if regulatory approvals could be challenged with a change in administration?

That's like someone is approved for a license to own a gun, then that person uses the gun to commit a crime, and then their license is revoked. Then someone points the finger at whoever it was that approved that person for a gun license.

What would be the precedent if a merger approval gave you a free pass to be anti-competitive?

I suppose the question then becomes "should the things that FB did with WhatsApp and Instagram since acquiring them be considered a crime"?

I could see eyebrows being raised around interop work, but I'm not sure about the rest.

The FTC is an executive agency with the enforcement equivalent of prosecutorial discretion. It's not a court.