I think she would have stayed with her. It was true love. Why they saddled her with the superficial Kennedy later on is still a mystery to me. I guess Whedon really did like pretty ladies, but it was shocking how lacking in substance that character was. The line readings left a lot to be desired, esp since AH is such a strong actress.
While I agree that the Willow/Kennedy relationship is written very shallowly, I don't think that it's shallow because Tara is Willow's One True Love that she can never move on from. The entire point of introducing Kennedy is to show that Willow is, eventually, able to love again; Tara
was her true love, and Willow can't replace her, but that doesn't mean Willow should have to always exist in endless lonely grief. The Willow/Kennedy relationship is shallow because the writers weren't interested in developing any of the Potentials as characters beyond what was absolutely necessary, which in Kennedy's case meant establishing at least the idea that she is attracted to Willow, though the execution leaves a lot to be desired.
In spite of all their worldliness, Willow is an insolvent college student who is likely dependent on her parents and lives in temporary college housing. No court in the world is going to place a violent and erratic patient who needs 24 hours a day car in her custody on the grounds they dated for about a year.
Well, the courts left Dawn in Buffy's custody while Buffy was unemployed. No income, no health insurance, no plan for paying the bills after Joyce's life insurance ran out. We know that Buffy is dead for most of this and that Willow and Tara were actually raising Dawn (also while unemployed college students!) and just convincing people that Buffy was still around by using the BuffyBot, purely because the alternative is that Hank gets custody... Hank, who is an awful parent, sure, but who actually has a source of income. When Buffy comes back, the job she gets is a minimum wage fast food place; there's no way that was actually enough to be supporting her, Dawn, Willow, and Tara (since the two of them were still living there without jobs).
We get one episode where Buffy makes a really bad impression on the social worker, but her financial situation alone should have been enough for her to be deemed unfit. And this is when dealing with Dawn, who is a minor. The social workers of Sunnydale clearly don't make the best choices, and it's not like Tara's family would have fought Willow over being Tara's guardians.
Do 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.
She would never have given up hope of it being temporary. Buffy
dying wasn't enough for Willow to give up hope. I think Willow probably would have dropped out of school with the assumption that she and Tara would be able to go back together. She would drastically underestimate the amount of care that Tara needs, but she
would be willing to do it, I think.
Probably, it would become a situation that all of the Scoobies have to participate in. Willow and Tara probably do still move into the Summers' house, and then Willow has to drop out to take care of Tara full-time. Maybe Xander and Anya move in, too? This would be meant to be temporary from the beginning, but, in the short term, there need to be people who actually have jobs to help support this household. I always wondered why they weren't the ones who moved in instead of Willow and Tara in the first place; the Scoobies were trying to decide who should take care of Dawn and picked the unemployed dorm students instead of the engaged couple with jobs?