I don't think Buffy came back wrong, nor do I believe she came back part demon. The latter can be disputed by season 7 when she enters the alternate dimension and discovers how the first slayer was created. Those men literally placed an aspect of the demon inside the First Slayer and offered to do the same for her. Why would they need to make her less human if she already came back part demon? And to be fair, you could argue the Slayer is part demon because of this lineage. If that power is passed onto the next generation, then technically you could argue the power is partially demonic in the first place. And we never saw anything to indicate Buffy was stronger after she came back, so her becoming part demon? Nah.
There are plenty of theories as to why Spike could hit Buffy with no pain, even with the chip still in his head. He spent months without Buffy, and technically grieved after her death, so on some level, it could have altered the chip's function. There are plenty of possibilities, but Spike saying she came back wrong was clearly a jab, nothing more.
To me, the changes in her personality reflected the times. In season 6, we see her actions reflecting her emotions. She feels dead inside, therefore she turns further into darkness to hopefully feel something; anything. It's destructive behavior at its worst. In the process, we also see her starting to grow up, and quite frankly, growing up is a painful process. She's coping with being brought back to life after she was literally in heaven. She's her sister's guardian AND is struggling financially, meanwhile her best friend is also dealing with a form of addiction. Her father figure and mentor is gone. Personally, I think she turns to Spike because everyone else is dealing with their own problems, but in reality, she's blind to what is going on around her.
Later in the season, we see her end things with Spike. She tries to be there for Dawn after discovering how miserable her sister is and that she has been stealing constantly. She tries to help Willow too. I don't think she came back wrong, I think she feels wrong because life kept moving in her absence and she got thrust back into it AND was expected to grow up quicker. It's different from the type of hardships she faced in season 2, which I also find incredibly hard, but more so coming-of-age realities than the harshness of adulthood.
By season 7, Buffy has learned a lot and hopefully from some of her mistakes. At this point, they have faced enough battles and apocalypses for her to understand that battles have casualties. Do I think she was willing to sacrifice Dawn? Absolutely. Her having Xander drive off with her doesn't dispute this at all. If anything, I think she cares about them, but also realizes neither of them are particularly useful in the final battle. Dawn isn't a potential. Xander lost an eye. Even when they come back because Dawn turns the car around, Buffy cracks a joke. "If you die, I'm telling." Would she grieve over the loss of her sister? Yes, but in some ways she's already faced that reality when she sacrificed herself for Dawn. She chose the alternative.
Which brings me to my next point. In the end, if they don't win the battle, then what is the point? Buffy understands lives will be lost. The impending battle they're facing is unlike any they've seen before, and if they lose, then even more lives will be lost. One life - even the life of her sister - doesn't outweigh the many. It's also why she is nonchalant about Chloe's death too. And you could argue that Buffy jumped in to save Dawn, but at that point, Buffy had already lost soo much. She considered Dawn an innocent. Closing the portal only required the loss of one life to save the world. If Dawn died, the battle would still continue on and other people were going to die. It's a matter of a bigger picture.
Even when Angel swoops in with the amulet, he offers to wear it. Buffy rejects this idea, not because she loves Angel, but because she can't sacrifice him in case they lose the battle. She clearly thought about the possibility of them losing, and by having Angel back in LA, it gave a second front, someone to continue fighting in their stead. And as much as she cared about Spike, I don't think she particularly cared if he died either.
Personally, I think the show did an exceptional job of having Buffy grow and change throughout the seasons. As someone who watched all 15 seasons of Supernatural and constantly witnessed the back and forth between Sam and Dean sacrificing themselves for each other, and putting the world in danger because of it, it's refreshing - that even back then - we had someone who learned and grew as a person.
