The stripping of the protection of True Love requires Justine or Thomas to be unfaithful, you presume Justine but Thomas is known to be unfaithful  with Evanna which would make a lot of sense.

I cant remember what book it is, but essentially Thomas and Justine have an arrangement. So that he can be with her without burning himself, because her love for him and his for her, will trigger the burning. He sleeps around, with Justines aproval, and sometimes with Justine as a involved party. It was a conversation between Harry and Thomas.

Really. I would love a link link, please.

There is a recent interview where Jim hints that a common backstory could be applicable.  He answered that its possible for him to do a Tavi and Harry book with Tavi as the thief and Harry as the wizard.  So I think in Jim's mind its entirely possible for them to exists within the multiverse he seems to be building in Dresden Files.