True. Like you said it does work better when Ian did the threesome with Louise since he will mention it during their flirting scene if that was the case.I think it admittedly works better in scenarios where Louise was part of a threesome, or isn't in relationship with Jeremy anymore. But even otherwise, we know Louise is very attention-starved and has submissive streak a mile wide, so i can see her getting carried away with a suave/dominant Ian who can pass the check(s) for the options to get there.
Regarding Louise being faithful girlfriend, note that she can be easily convinced by Lena to have a threesome with Ian or Mike even if she's still in relationship with Jeremy, so you could say where she draws the lines can be quite flexible. Like you note, what people expect from others and what they do themselves can differ significantly.
And yeaaa she has shown to be submissive, calling her Lena mistress or letting Jeremy have her way with her. I guess she just draws the line differently like you said, and I can tell myself that Jeremy was probably neglecting her so she was extra horny and attention starved. At the end of the day Im all for Louise content. She's hot af and imo has the best dressing sense out of all the grills. Eva nailed it with the consistency when it comes to Louise. The black tank top and panties combo will never not be hot.