Okay, Dawn is made of Buffy. I have no clue how Buffy got to this conclusion but for the purposes of the argument let's accept it. Dawn has a human component "inherited" from Buffy and a Key component "inherited" from her days as a ball of energy. He blood opens and closes the dimensional portal because of the Key component, not the Buffy component. Hence, for Buffy to be able to close the portal she has to have a Key component in her. But while crazy people see Dawn's "Keyness" they see nothing of the sort in Buffy. The monks don't bother to inform Buffy of her Key-ness. What exactly makes Buffy believe that her sacrifice has any chance of working? That's what bugs me.
Compare with Becoming. The situation is quite similar but back then Joss made sure that we knew why Buffy made her choice. He used Whistler as a plot device and made him tell her: "Then only Angel's blood will close it. One blow will send 'em both back to Hell.". So there was no wondering why Buffy does what she does.
Compare with Becoming. The situation is quite similar but back then Joss made sure that we knew why Buffy made her choice. He used Whistler as a plot device and made him tell her: "Then only Angel's blood will close it. One blow will send 'em both back to Hell.". So there was no wondering why Buffy does what she does.