I think this is a situation that requires parental leadership. As you've discovered, young children's decision-making is not generally guided by wisdom and experience and long-term understanding. It's very much rooted in feelings of the moment and day-to-day considerations. I am very much in the radical unschooling camp, but I believe that it is possible to saddle children with too much responsibility, and that can work against them developing confidence and persistence.
I've found that at times putting responsibility for decisions and persistence upon my kids, and taking my kids' misgivings too seriously has undermined their own confidence. If they balked at something that they felt confident about a few days ago, and I allowed them to waffle on following through, they interpreted that as "mom doesn't think I can hack it either." Or "mom is also confused about whether this is okay for me or not." Or "I don't feel ready to a decision on this -- I don't know what to do -- and my mom won't help by deciding for me."
I am a reluctant leader. I tend to sit back and say "well, what do you want?" and wait for others to make decisions. But I've gradually come to realize that there are times when children draw comfort and security from their parents' decisiveness and confidence. They don't need their autonomy taken from them, but they need others to make bigger-picture, longer-term decisions on their behalf, taking into account their needs and inclinations. I would encourage you to do so with the schooling / homeschooling issue. Make a decision on your child's behalf, taking into account her needs, the reality of the options you have, your beliefs about what is best for children like your dd, and your gut feelings about what makes her tick and will support her most thoroughly in the next year or so. I'm not saying that parents should have the authority to over-rule their children's desires, but that parents have a role in providing the leadership that will guide their children through various moods and waxing and waning motivation.
So don't ignore her day-to-day feelings if they are at odds with the choice you've made, but express confidence that she will thrive, that the two of you can find solutions, that things will work out, that she can make things work. Obviously, if things get very dire you may need to adjust the choice you've made on her behalf, but I think there's a lot to be said for modelling persistence, commitment and creative problem-solving.
One option would be to decide that for 1st grade you will homeschool. Explain that she knows what school is like, but she's had mixed feelings about it, and schools clearly aren't happy about having children miss days -- especially not as they get older. She hasn't really experienced home-based learning, and you will spend 1st grade exploring that possibility with her. You know she is a bright, wonderful girl who learns well, and you know she will be fine in either situation. You just want a chance to see over the course of a year whether homeschooling fits her and your family better than school has. "This is a great year to give homeschooling a go," you can say, "because you will be at home when the baby arrives, and we can really enjoy getting to know our baby together." And then work hard to make homeschooling a successful and enjoyable experience.
And you know, I really believe that having a newborn makes for some truly awesome homeschooling possibilities. It forces you to think outside the box about learning and living, it allows your older children to grow into new levels of responsibility and autonomy and it gives you a way to really bond as a larger family. I have administered the New Baby Curriculum
to my unschooled kids twice (I have four kids spread over nine years) and it I can't imagine what it would have been like to be trying to get reluctant kids to school while dealing with a newborn.
Miranda