Two second summary: answer all the immigration officer’s questions, even if he’s being an ass.
On Wednesday my dad and I flew from Sydney to Los Angeles in Qantas first class. My dad has a German passport, while I have both a German and US passport (though of course I use my US passport with Global Entry when entering the US).
As we deplaned I explained to my dad that he’d have to go through separate lines, and that I’d wait for him at the baggage claim belt.
As we entered the immigration hall I saw that there was virtually no queue, so figured he’d be out in no time. Our checked bags arrived (yes, at this point in our trip we were checking bags), though there was still no sign of my dad.