I think you may be vastly overestimating the extent to which even that AU-Harry is willing to be "official" anything to Marcone. (Actually he's probably even better at avoiding it than canon-Harry, since he's successfully been hiding from the White Council since he was a teenager.) I mean, they string out the "plausible deniability where we all pretend Marcone doesn't know Harry is faking it" thing for a ludicrous length of time, and then it morphs into "Harry is a detective who is undercover as their fake nanny", but he never becomes their actual official nanny *technically* by the point at which the girls start trying to get them to make a different aspect of their relationship official instead.

Probably the closest he gets is after they finally manage to track down where the Black Council has been holding Kincaid (which was really the main point of the whole charade in the beginning, Marcone agreed to help Ivy find out what had happened to Kincaid, and needed to acquire better magical help to fulfill his end of the deal) and Kincaid is like, I'm fucking retiring after this, you can take over my contract with the Archive, and Harry is like, wait what.

But even that was officially bodyguarding, not nannying.