Could Mab get through the ward, the threshold, and have the knowledge and skill to tinker with complicated mortal magic? While treating Lea at the same time?
Easily. Lea has expended significant effort on making sure Harry's dwelling maps to her garden when making Ways between earth and the NN. Mab can just step through, inside the wards and the threshold, and do anything compatible with behaving as a good guest (which also explains why, if she was there, she didn't destroy Bob ... although I suspect she's more interested in keeping him scared enough to lay low and stay quiet about the immortality loophole than sincerely committed to destroying him).
And mortal magic is mostly similar to fae magic. The ability to teach him to be far better at it than he is was one of the perks she dangled in one of her earlier offers to try to recruit him for the Winter Knight. For that matter, she's probably far enough ahead of Harry to seamlessly disarm his wards and go in through the front door, if she wanted to.