The only way we will see Maeve again is if Harry is involved in some time travel shenanigans. However, Proven Guilty is exactly the book where that would seem a reasonable possibility, so your hypothesis may have some merit; but my guess is you've missed the mark by a wide margin. Meaning, while I think that it's possible Maeve was involved in the events of PG, I don't think she was pretending to be Sandra Marling.
Remember the amount of time and effort that was needed by Sandra Marling to set up Splattercon and to draw Molly into her sphere of influence. Maeve was far too lazy to do either of those two things, but more than that she despised mortals and wouldn't lower herself to pretend to be one for any extended length of time, plus I seriously doubt she would have had the skill set needed to organize a human endeavour such as a Con; and finally, she had no reason to want to corrupt Molly. Mavra or even the Denarians are better suspects for attempting to corrupt Molly.
To reiterate, it wouldn't shock me if we discover Maeve was working in the background during PG. In a time travel story we might even be shown how Mab learned her daughter was nemfected. Jim might do this to explain exactly what and why Mab did what she did, before and during PG.