Tara's mother is dead and Willow knows it and her father is a lying abusive misogynist. You think after Family, that Willow would ever have any contact with the MacLay's that's nonsenseDo you really believe though that Willow would quit school, and make the sacrifices necessary to care for Tara full-time long term? Or did Willow think all along that she could reverse the damage, and Tara would return to her former self?
There is a big difference in doing something difficult if one believes it is temporary then if one knows it's permanent.
I truly believe that if Willow couldn't have reversed the damage that Glory did, she would have called Tara's parents to help, and I'm assuming they would have put her in a home, or worse, an asylum.
Willow had already downgraded from internationally famous institutions or Ivy league to University of California for Buffy. She would move to some sort of online courses for Tara. Hell, she might drop out. I've always wondered if she actually bothered going back to college post season 7