Something occurs to me here -- I think fans carry around a resentment toward Joyce for her missteps regarding Buffy -- in Becoming and Gingerbread, most notably -- that Buffy simply doesn't. So I think sometimes fans miss how important Joyce really is to Buffy, even though the show makes it obvious. Their own emotions get in the way.

with the nonsensical development that Buffy's blood will do just as well as Dawn's to close the portal

I always read the success of Buffy's sacrifice to close the portal as being partly driven by Buffy's sudden and absolute conviction that it will work. Magic has always been shown as having an emotional and metaphorical component as well as a technological one. And we've seen enough evidence of spells not working quite as the casters intended for me to be okay with that.