Starting to sound more and more like SA Eagle...

Seriously, you're going to wear yourself out trying to engage them yourself.
Yes, there is the matter of apportionment.
No, they are not going to make a fair apportionment (if there is a case for it) without being bludgeoned to it.

You know they're screwing you. And there's only one profession equipped to fix it.

Hand it over to a lawyer.