Nathaniel, it looks like you need to clarify a few things for your mother--well, you know that, so let me just say DUH for stating the obvious.
Clearly, she needs to know that there's a lot of info on the Internet that isn't accurate and a lot that doesn't apply to you. That you have to jump through some hoops before you can take any action anyway. That you're not so impulsive anymore. In fact, if she automatically assumes that you're going to leap right out and "mutilate" yourself, she's the one who is being impulsive and jumping to conclusions. Et cetera.
So...can you write up another letter to give to her? Perhaps one that includes a reasonable timeline that shows a SLOW progression of events and that indicates that you have every intention of professional consultation/approval at every step? Or would that mess her up more by making things concrete?
I don't think you're going to be able to control her Internet research. You said yourself that it was neat that she was doing it. And she will eventually run across information about phallo. If you act quickly, you might be able to bring it up first and explain that you're not going to run right out and have a penis fashioned. But I don't know that bringing it up first is necessarily the best idea if she's having a hard time now. On the other hand, if she does the research and you've said nothing about it, that could be bad, too.
Okay, maybe a letter with a timeline and a statement at the end that certain changes are all that you're thinking about right now, that you don't currently see yourself going beyond those changes.
You don't want your mother coming to Susan's as a significant other, do you? I mean, it might help her, but it might make you very uncomfortable.
Nate, I think you should strongly consider asking your therapist help you with this, if you haven't already. It's a lot for you to handle, and we at Susan's can only help you so much with other people. And I think it's something you need to at least try to work out before you move. You don't want to leave your mother stewing over graphic Internet photos while you're off living your life and doing (from her perspective) who knows what to yourself.
I think it's a very GOOD sign that she's starting to confront this, by the way. I'm sure it feels like hell, but progress often does.
Hang in there.