she's dripping in so much magical shit she could live for forever and probably not even actually age one day. seriously, she's a witch, her dad's a wizard/warlock, her mom's a witch, then there's the demon crown, blah blah blah blah blah
AREN'T POINTLESS RANDOM TANGENTS FUN?
anyway. The Doctor might have been playing a Xanatos Gambit to keep Quote away from the core in the case that he was defeated. perhaps he theorized the crystal COULD keep him alive, but not if Quote blew up the core before he (Doc) could reawake/form/gather/whatever and stop Quote, and so ordered Misery to delay him.