Since I devoured the whole thing in one sitting, it's difficult for me to sort out if my initial reactions are fully baked or not... but yes, I agree, this doesn't hit well for me at all.
To a degree, the soft-retconning
could be attributed to Harry learning more about how the world really works... but in this case I can't buy it. It's too different from what we saw Harry go through after Susan, or even the way we saw Lara get burned
one book ago, and I
really hate throwing doubt on Murphy's feelings after she died. We got books and books of build up between those two, it feels insincere to even
imply "sour grapes" now. It's not just kicking someone when they're down, they're
dead, and that just seems mean, in a way the saga typically hasn't been before.
I'm not all that committed to any ship but even if Harry winds up with someone else by the BAT, it doesn't mean that what he had with Karrin wasn't real while they were together, just as what he had with Susan was real at that time.
IMHO, any of the alternative explanations in this thread would have been better than what we got in the book.
It's one thing to have an unreliable narrator (which Harry is), but too much backtracking and retconning can damage a reader's faith in the narrative, creating distance and doubts, and that's not a good thing. Yes, maybe Lara can pull back on her power to some degree, she's probably old and powerful enough to have that much control, but between that
AND the oh-so-conveniently waning protection
AND Mab's catty little swipe at Murphy, it's all too much.