Justine, it breaks my heart to hear how your family has been treated.
I can imagine that you can't get this out of your mind. And yes, I do think your boy has been wronged by the sounds of things.
Kicking, biting, hitting, throwing things are very very normal behaviour for a child 18 months - 36 months old. Some do it more than others and maybe that creates an illusion that some mothers think that "they must be doing something more right than the other moms who's kids behave like that". Sure, I won't like it if another young todler bites my child, but I will not be rude to the child or mother. That is not cool.
If you don't want your child to get hit/bitten by other kids, don't expose him to toddlers who are at the age of doing that.
The fact that the leader confirmed that his behaviour is normal should be reassuring as well. But, if you do fear that the behaviour is really extreme, a bit of play therapy might help. I have read in ALL books etc that it so normal for them to behave like this at this age. They don't have the comunication skills yet to articulate complex emotions.
I read somewhere that a 2 year old is like a blender running on full speed with the lid open.
It really saddens me to hear other moms treated you and your nanny and son like this.