I am an adult trans woman and a mom. I love Chrys's answer and your supportiveness, but I just wanted to provide a little more insight here.
Is it okay to encourage [...] genderqueer and transfeminine as alternatives to transgender?
Transgender identity is inclusive of genderqueer and transfeminine individuals, so it is important not to contrast these with each other. Perhaps you meant as alternatives to identifying as a binary trans woman? In any case, the difference between these identities should not be defined by what physical changes a person chooses for themselves, but by how they feel internally. It is certainly great for your teen to be aware of all the different possibilities, but I want you to understand that at the end of the day if your teen identifies as a woman but doesn't want to change some physical features that is ok.
My oldest child (17) told us that he has been struggling with finding his "presentation" and he "thinks" the answer is that she's a woman.
While there is likely lots of questioning going on, I would discourage placing too much weight on your child's choice of the word "thinks". Your child may have trouble expressing certainty because of the fear of what kind of responses she may receive (i.e. "Are you sure you're a woman? You don't do X."). One of my biggest fears when I first started trying to express my gender identity was that cis women (even liberal, accepting ones) would feel I was claiming an identity that I didn't really represent. For now, from what I gather, it seems your child has chosen she/her pronouns. That could change, and that's ok, but you should use what your child currently feels like. For that reason, I'm going to refer to her that way through the rest of my answer. In case it's not clear, this is why I'm gathering she's currently chosen she/her: "the answer is that she's a woman" and "Oldest is searching for the right female name and wants to change her name in time for senior year".
We are lucky to have a children's hospital close by that has a specialty gender clinic and I called the next morning and got the ball rolling there. Oldest will have access to expert counseling as well as medical treatment should that be the route of the future.
That's fantastic! And your daughter is so blessed to have a mother willing to do this for her. I would also encourage you to get counseling as well, if you haven't already, because it can help you process some of these worries openly before you decide whether/how to approach your daughter about them.
Because I don't want the full gender change (feels like a rejection of the past 17 years of our family life), I suggested that s/he look into being genderqueer or transfeminine.
This is the area of biggest concern to me, and why I would strongly urge you to consider counseling as well. While it is perfectly normal for you to experience feelings of fear/rejection/loss over these types of changes, the way you respond to your daughter will have a huge impact on her well-being. If your daughter does feel the need for hormones/surgeries (there are many options and whether they are needed depends entirely on how your daughter feels), then your expressed reluctance may make it harder for her to accept or communicate to you. She may not know or want them right now, and that is fine, but you should be careful about discouraging her from them. If there are concerns about timing, that is something you should definitely bring up with the therapist.
should I just keep avoiding names and pronouns
Chrys already covered this pretty well, but you should use whatever name and pronouns your daughter is comfortable with in the moment. You don't have to wait for a legal name change to start using the new name. However, if there are people she is not out to, you should have a discussion with her about how to handle referring to her around them.
Bottom-line: Chrys is absolutely right that no matter what bumps in the road there are, you are on the right path! I just love the way she finished her answer:
Your child is lucky to have supportive parents, and if the worst experience ever is that on Day 1 someone said "are you sure you need to go 100% all the way, you might just be genderqueer?" then that is going to be a pretty great transition. Smile, be glad you're someone who is trusted to be along on the journey, and gather your strength for protecting your child from a world that won't all be as nice.
Thank you for reaching out for help! I love that you care so much about your daughter to consider all these factors!