look, good table manners are about eating tidily, if you can, asking nicely if you need something, ie salt. waiting your turn. Asking if you can get down from the table politely.
For a more graphic example, love, my mother battled with me about using my left hand for eating my soup (despite already having a left-handed daughter!) she finally gave up.
My son is dyspraxic. he has beautiful manners, and is right-handed, but has such poor muscle tone he can use whatever bloomin' hand works best for him.
The OP, who has gone, surely knows that to bully and isolate a child, refuse to eat as a family over something so trivial, is not aout the child , its about the adult playing out some mean and bullying scenario towards the OPs child.
I don't believe it for a moment, but am shocked that some people would focus on the child rather than the adults lack of care, compassion and family sensibility.