Isn't it a little out of character for Harry to blithely assume that Hermione can't possibly die in her dementor mission? He doesn't even know how Horcrux 2.0 works--is there any good reason to think that the Horcrux will preserve your life if you deliberately fuel your magic with your life to kill dementors? (It's basically just a body-hopping spell, not a life-preservation spell.) Would a horcrux restore to Harry the life and magic he used to revive Hermione?
It just seems suspiciously out of character that Harry has now suddenly turned into an optimist with regard to Hermione's survival. He even says to himself he would never let her risk the mission if he thought it was actually dangerous, which means that he apparently actually fully buys into her immortality.
It will be tragic for Harry if she is dead again, for real, next week. Not because death is tragic per se, but because it will utterly blindside him.
I agree that it's worth the risk, but apparently Harry doesn't.
'"I thought..." Hermione said. She sounded uncertain. "I thought for sure that after this, you and Professor McGonagall wouldn't... you know... let me do anything the least bit dangerous ever again."
'Harry said nothing, feeling guilty about the false relationship credit he was getting. It was in fact the case that Hermione was modeling him with tremendous accuracy, and that if not for Hermione having a horcrux, the surface of the planet Venus would have dropped to fractional-Kelvin temperatures before Harry tried this.'
I agree with you that unicorn blood is more likely to be significant than the Horcrux in this scenario, and until this last chapter was posted I expected HARRY to think the same way, which is why his thinking stuck out to me as memorably optimistic.