What do you think about the cutting room bit? Was David being mean about Gillian or was he trying to hide his real emotions towards her?

Being mean? About the key thing? That was just basic Gillovny banter. I think that’s exactly how it happened. She asked what was the key and David made fun of her as if she knew anything about that and they both laughed over the phone, she probably told him that she knew better than he did and he agreed and that was it.
That’s not being mean, that’s just Gillovny being Gillovny.