Pronouns isn't something you need to use with a person, it's for when you're speaking about them to others. Easily avoided when speaking with the person, which is what I'd do. I'm GC so I don't support pronoun bullshit. I don't have a problem with anyone wanting to call themselves anything, I'm just not going to agree that it's true that's all. I could eg mention to a neighbor that DC prefers to be referred to as "he" because DC believes they're trans. That's just stating DC wishes and beliefs, I don't have a problem with that. Everyone is entitled to their own wishes and beliefs. I personally wouldn't be referring to a woman as "he" though. If she goes as far as the procedure known as gender reassignment surgery, I'd be happy to consider her a man (but not male, she has the wrong chromosomes for that) and refer to her as "he". Anything less than that I'm seeing it as nonsense. I realize this view may offend but it's my personal beliefs which I'm entitled to, so I'm not interested in getting into any discussions about that with anyone.
Name isn't relevant to sex. People have all sorts of strange names these days, it's not all traditional names of the country of birth is it, so not much difference between having a nickname or an unusual first name. Rejection of the name you chose, I understand it hurts, but even without trans being part of the equation, a person can change their name, go by a nickname/middle name/shortened version you don't like etc. A person making their own decisions is part of growing up and must be accepted, especially if you want to continue having that person in your life. So I'd use the new name, I wouldn't have a problem with that.
Your hurt is yours to deal with 💐, she's not trying to hurt you that's the thing to remember, she's doing what is right for her and you just happen to be hurt by it. Same as she'll be hurt if you turn down medical fees for her to do something to her body that you don't want her to do. You're not trying to hurt her by doing that, you're just doing what's right for you, she needs to understand and accept that. Just as you understand and accept her choices. I don't like the emotional blackmail about not being in a room with you, not being around you etc. That's manipulative, threatening to cut you off if you won't do what she wants. Not nice. I wouldn't let manipulation influence my decision. I would be pissed off at attempts to manipulate though and would make my feelings known, because such behaviour is out of line IMO, manipulation is an attempt at control.
Regarding safety, your DC is an adult and is free to make her own decisions about safety. Acceptable risk is a personal decision. Some of us are more risk averse than others. Neither way is right or wrong. It's something you'll have to accept, although you could raise the issue with DC to ensure she's aware of the risk and has considered it. Same with dating difficulty. It's her decision to put herself in that situation. Again, you could raise the issue to ensure she's aware and hope she considers it before making a final irreversible decision. It's wrong to try to control another adult. She has the right to make decisions, even those you may consider to be bad ones. Your feelings about it all are yours to deal with. Your feelings aren't her fault, she's not doing this "to" you, so your feelings aren't her problem to fix.
Are you going through a kind of grief perhaps? Keep in mind she's young and could yet change her mind, either about being trans or about surgery/hormones. You haven't lost her yet, but she's growing up and changing dramatically, I wouldn't blame you if you felt you needed some counseling to come to terms with it and process your feelings. It must be a shock, even if you suspected before. It's a lot for you to deal with.
I'd ask her what does being fully out during college mean to her, what does it entail? Ask her to help you understand what it is she means. It's not judgement, it's clarification. At the moment you're worried, but you don't know about what exactly and you're making assumptions, because she hasn't told you. She's also making assumptions, assuming you know what she means, but you don't. So find out. That's the first step. Communication, compromise, acceptance and respect, on both sides, is the key to successful relationships. You can remain true to yourself whilst respecting your DC as a human being, even if you don't agree with some of the decisions DC makes.