But none of that is relevant to the fundamentals of Thomas and Justine's relationship. Yes, at one time, Justine was protected from Raiths by the power of True Love. That was an outgrowth of the existing connection between Thomas and Justine, plus some mystically-significant qualifications. The mystic protection was broken in the GS epilogue (either temporarily, as hoped for, or permanently), but there's been no evidence that this shifted their fundamental devotion to each other. Thomas lives and breathes for Justine, and the fact that she had True Love protections is proof that it was reciprocated on her part.
Yes, their relationship could have downgraded to the point that Justine no longer loves Thomas, and would willingly betray him to his death. There is no evidence that this actually happened, however. We're back in "Harry is secretly a serial killer" territory.