Daas Yochid,
I clearly am missing some information about this class of people that is driving your argument.
The Kusim are a very interesting case (as far as I know unique). The King of Ashur appointed a Kohen (who had been living among the aseres hashvatim) to be megayer them. The Kohen didn’t have a choice in the matter (imagine an Orthodox Rabbi with a gun to his head). Still whether or not their Geirus was valid was a machlokes tannaim.