Point of order:
A month ago Dresden used Charity's blood to find Molly in Proven Guilty canon; that was how he realized she was in the NeverNever, and tracked her into Arctis Tor. And in this case, Mathews has been acting in loco parentis for her, strengthening the sympathetic connection.
It should work just fine.
Just that it occurs to me that scrying the Mercedes or, better, Black Rider would probably be even more effective.
Use Black Rider as a focus, and ask the Crown to give us the current location and previous 24-hour movement history of everyone who has been inside the Mercedes ever since we hit it with HMP.
Then cross-reference with Black Rider's movement history.
Still gonna need to go to the Carpenter household, to get Lydia armored up. Molly might get a quick bleach bath in the interim.
QUESTION
Is the Mercedes still at the tattoo parlor?
Magic is not automatic, you have to make the rolls and in the case of scrying the more degrees of separation between the focus and the target the harder it is for Harry to pull it off.
By this point it is almost guaranteed Katrina found another way out of Undertown or maybe took a Way out