When we checked in AKL concierge my MIL was NOT present. I actually had forgotten about that rule until I read this thread.
We did ask several questions to be sure she would get checked in properly and upgraded her 1 day base ticket to a 2 day park hopper, so I'm sure they knew she wasn't a "phantom" MIL.
Her not being present when we checked in and got our keys was not mentioned, though.