1. Xander was perfectly justified in telling Buffy that Willow had said, "Kick his ass" when Willow had been working on re-souling Angelus. At this point, Buffy would have hesitated and waited for Angel to return and probably would have been killed, along with the world.
2. I like Kennedy. She was the only one to want to be a slayer and she offered to be bait to capture a bringer. She interceded in Willow's life when Willow looked like she was never going to have another love after Tara (and I have seen this happen in real life) and although she was not the final answer for Willow, she was the right person to come along at the time. She was the one Willow entrusted to get the scythe to the slayers because she was the one most likely to get through, just as many armies chose their best soldier to be the flag-bearer in mediaeval times.
3. Buffy was not abusing herself nor was she incapable of making a rational decision in Season 6 when it came to Spike. Spike was the least judgemental and most perspicacious of her friends. And uninhibited sex that destroyed a building? Why wouldn't she want that?
4. Buffy, the successful general of Season 3 who organized bringing the mayor down had gone through a headlong deterioration by Season 7 where McGuffins were required (the scythe and the medallion) to defeat the enemy. In a rational plot, she and the potentials would have been dead. The show could have been called "The Rise and Fall of Buffy", just as Ats could have been called "The Rise and Fall of Angel".
5. Angel never signed away the Shanshu prophecy because he signed it "Anger" and since that was not his name, it was null and void. But you can't sign away a prophecy anyway - it's not a contract that you can waive.