I think the true love protection is not removed by not loving one self, but is put in a non-functional state. Perhaps especially in this case where Harry does not feel he is worthy of Murphs love, because he feels responsible for her death. So if the kiss is not enough to loose it, perhaps it is back by the end of 12 months, now that he can love himself and feel worthy of love.

When Susan becomes infected at the end of grave peril, book 3, it is not untill blood rites, book 6, that we know he is protected by true love. There is no way to know if he was protected when he was at his lowest in the year following grave peril.

Regarding Mabs comment about it happening twice. I would have to listen to it again, but I think it referred to how it was the second time that felt responsible for the death of his love (third time if we count Lash, and for a long time je also thought he killed Elaine). That problably has an effect on whether or not one feels worthy of being loved.

Regarding the rarity of true love protection. As far as I remember the only place the rarity of it is mentioned is Thomas saying something along the lines of "it is rarer than you think" when him and Harry first talks about it. I think that also has something to do with the humans the white court interacts with. If they targeted normal couples living like the Carpenters they would problably encounter it more often, but they target young people at night clubs and powerfull people.

I'll be surprised if we get any explanation that's satisfying.  Well, beyond a long internal monologue by Dresden that rests on a foundation built on a reading of 'All You Need is Love.'  I'm not expecting the readers will get anything close to an explanation of how the universe Butcher has built all works and comes together logically (even one that's at the descriptive/predictive/Newton level and not at the fundamental/'why does it work that way' level). 

After Twelve Months, I have a creeping feeling that the conclusion of the Dresden Files will reveal that the entire series was 'True Love Conquers the Outsiders.'  If that's all there is, then let's just keep reading and hope for a rollicking, enjoyable remainder for this yarn even if at the end we're left with a lot of Lost-esque unresolved loose threads.