(1) It has been confirmed by Scott Allie in the recent Slayalive interview that Twilight affected her in #34, and is presumably still affecting her. This effect caused Buffy and Angel to act in such a way (space sex) that led to the birth of a new reality. It also may have caused Buffy to believe her heart belongs to Angel. (And vice versa.) This may have been done to ensure a bonding between the new "parents" so they will stay together and care for the "seed". When the Seed is destroyed, in Sunnydale, this artificial bonding will also be destroyed and everybody will be themselves again.
(2) Something may have happened to Buffy's heart already way back when the Hellmouth was destroyed in "Chosen". Somehow a part of her heart was left behind there, maybe because Spike burned up there. After all, he was "in her heart" at the time. So the part of her heart where he was either has a big burn hole in it, or has been left behind in the Hellmouth. Having a piece of her heart missing could explain Buffy's OOC behaviour during S8, such as robbing banks. It might also explain why Spike wasn't in her memory cubes or why she never spoke about him or tried to contact him, and her cold treatment of him in #36. To remedy Buffy's "heart problem" they're going to have to retrieve the piece of her heart that was left in the Hellmouth (or somehow make sure it was never destroyed in the first place by means of time travel).
And striped_lily on Buffy Forums posted this brilliant piece of comparative art, showing the similarities between "The Long Way Home" season opener and the finale "Last Gleaming"--both written by Joss.
The comparison that seals the deal for me is Buffy exclaiming, "How does this happen? How do we turn into twelve-year-olds? Every time we talk?" and how it's offset with the way Buffy and Angel have been behaving in Last Gleaming. Also remember in "Chosen" when Angel is acting jealous about Spike and Buffy says (in a very annoyed voice), "Oh my god, are you twelve?"
I also love the two silhouettes of Buffy living in her fairytale castle and Angel overlooking his domain in Hollywood--both locales are places of fantasy yet again.