I think the part I really don't understand is a non-exclusive mistress
I'd take that as to be understood that she is not monogamous; ie she's free to pursue other relationships. That's what being exclusive means after all; otherwise it'd be secret/ive or something xD
I don't know if I'd call a mistress to be more than a girlfriend; I guess that really depends on the context. In modern society I'd say a mistress is basically always meant to be secret (at least everyone pretends she is), a girlfriend isn't. Historically she could also be acknowledged, with extreme cases such as concubinage relationships. But I don't think this applies here. ^^
I'm seeing that it's been a while since I read the details about that stuff in the game and it's harder to judge from memory. I can only say from ingame context I never felt too confused but of course it's possible I just assumed I understood and got it wrong, dunno
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)