I read the whole thread, but only advice I could give is time. I can now have Sukei on my lap and absolutely no more biting. He comes up and snuggles now with me and Kasey, my German shepherd.
i have no reason why but time.
It seems to me greys do what they want to when they’re good and ready.
i hope your issue is resolved, mine seems to be.
I really hope that people reading these things understand a grey is not a toy, but a life commitment.