I am presenting an idea and my thought processes behind it. I made it clear myself that he has the final say whether it goes in or not, because I do respect his authority here. If, after I've presented my idea, he thinks it won't work, then I'll drop it. It'll be placed into my rather sizable library of ideas. I just had a cool idea for the RP and thought it might be appreciated. There's no other motive behind this. I'm not arguing here.
The mind control isn't an all or nothing thing. They can influence the persons thinking, talk to them in their head, and convey their intentions that way. How else can they make deals with the host? An idea for how they operate would be that they pick a semi-permanent host for themselves, someone they get along with and have similar goals to, to be their sort of representative. RPing one of them would essentially boil down to a sort of split personality character, except that the other personality is a completely separate entity.
Alright, we can change the backstory. Say they're a normally occurring race like the others that are being included. In fact, this would open up more options for character diversity, a greater range of character goals.