I agree with Elliemay and Moxie here. I understand how awful what happened to you was and think your ex-T handled it very badly. But she's made it clear under what circumstances (for whatever reasons) she's willing to hear you out. The letter seems to be what you would say were you to have the meeting you want--but which she has only agreed to within her conditions. Beyond that, because it overruns her boundaries, I doubt it will be heard in the ways you want.

I don't really see the problem with a mediated session, if you trust your current T. If not, then I wouldn't agree to it. But I suspect your ex-T wants the witness more for her legal protection than as any statement about your ability to conduct such a session. She seems to believe there's the chance she could be set up for some sort of retalliation, so she's being defensive. It's not that unusual, and needn't have anything to do with the reality of the situation as you see it.

I do think closure doesn't need to involve her at all, and I think you're right to be extremely cautious about your intentions and actions in order to protect yourself. Have you shared the letter with your current T?