Have you asked her why she is "speculating?" I'm not sure that's the proper term given the behavior you said he is engaging with you as an adult. It seems unlikely to me that he did not engage in this kind of stuff when you were a kid.

Even when you have recounted what she's said in the past like about your mother, you reported it more as she said she thinks its possible, or likely, and perhaps it is you who hears it as her saying she "knows."

There are a few reasons and probably many more than I can think of for her "speculation." First is that you are dealing with a very challenging situation that seems to be at a crossroads where you need to know/do/be something different than what has been going on. As you have said, you've gotten nowhere in therapy before this. If I were in your shoes, I would want to know my story and it seems like this T has at least moved you forward in some understanding about your family. And it makes sense to me that her discussing things with your mother seemed to be helpful and that's why she wants to talk with your Dad. But I don't think she's insisting that she "knows" the truth in a certain kind of way.

I don't know what her specific field is, but the world of social science (borrowing from science) follows a "hypothesis testing model" and don't get me wrong, I'm not claiming therapy is scientific in this way, but if the hypothesis is "maybe your father sexually abused you" (and this isn't the way a scientist would precisely formulate it), the question is how might you figure out whether this might be the case? The first is to "speculate" to you to see how it fits, and you're not running screaming in the other direction at the possibility. The next step might be to talk to the willing father-- and conversing with family members with client permission for their information (not to violate confidentiality or give him any information) is an accepted way to go about things. Therapists actually do this all the time, especially when people are in the hospital. Maybe she won't learn anything helpful, maybe she will. But speculation about what might be at the root of your very difficult problem isn't wrong, not at least as far as I could see. But you talking about it with her seems like a good thing, if you don't entirely understand it.

I don't have a dog in this fight about what you do or don't do in your therapy. It does seem to me that you've been making progress with this therapy, or maybe it's just coincidence that you've been able to reveal your father's current behavior towards you. I can't tell from your descriptions whether your therapist is a genius or an idiot, or somewhere in between; frankly I don't think anyone can. And your feeling "small" suggests that this has kicked off something significant that's about your childhood, and that seems to be a positive thing. Wish you the best.