I'm sorry if this is out of line (because I fear it might sound that way, mostly because I don't know the whole of your relationship with this t beyond what I've read in this thread) and I apologize if that is the case. It's not meant in any way to be disparaging or with judgement, just Something to think about and accept or reject, as appropriate.

From an outside perspective, you are setting up a dangerous dynamic with this t. You are already placing her in a position of great power over your actions. I'm not sure how much of it she is encouraging, or if it's something you do almost instinctively (I know I've automatically done similar things with certain people in my life, so I know the instinctive part can take over). As so many others have pointed out, you are giving her a lot of power over something very meaningful to you. If there is a safety concern around all this, I can see the benefits of holding off to open it in her presence (if opening it would potentially be so destabilizing that you would do something impulsive or dangerous after opening it and without reaching out), then there's merit to forcing yourself to wait, but I'm not getting that sense from this post. It would also make sense to open it in her presence if she provided some insulating factor (also mentioned above), but you say you don't trust her enough yet to get that.
I'm guessing t will not become angry and punitive (as a controlling parent might) if you choose to take the power back from her by opening the gift on your own. I think it might actually be quite empowering to make the decision and act on it without seeking her approval. her response to your email does not strike me as angry or upset in any way. I think there's a fair amount of transference or projection in your reaction to all this...
if she really is demanding you open it with her, I would consider another t. Even with safety being an issue, you are an adult who currently has freedom of choice.