How can I protect her if she won't let me?
That's pretty hard! And, no matter what she's thinking, or that it might be hard, it seems like the two of you ARE a team and the you have a right to this information.
Once upon a time, long, long ago, I was in a complicated but really good relationship. He was the first person to ever suspect I had "a problem" and decide to pursue it. The pursuit went awfully slow. Finally one night he directly asked me a few questions, which, it turned out, I couldn't form words to answer. (We went with "yes or no", head nodding.) I left that night, sure the world was going to end, he was going to hate me, or worse yet, pity me, and nothing good was going to come of it. I was kind of shocked when the sun came up the next day. More shocked when he treated me like he hadn't changed his mind about me at all. It took awhile to accept that that was true, not some kind of trick. It took a few years before I really got to where I was usually pretty sure he wasn't going to hate me for the next weird thing that came up. But, no one had ever loved me for just being me, and it's still pretty hard to believe that's a real possibility. Everyone has their own personal version of this stuff, but it might be a bit more skewed than you realize. (yet)
From a tactical, survival standpoint, it sounds like it might be a good idea to get some kind of feel for what these people are up to now. If they are still alive, for a start. From a practical standpoint, two sets of eyes are pretty much always better than one. Any idea what her therapist's take is on all this? Because I know of a Vietnam vet who spends a lot of time sitting in his garage with his shot gun, watching the property line, waiting for his neighbors to attack. (Which ain't happening!) One of the things I have trouble with myself is deciding if I'm reacting to a threat that is real and present or one that's long gone.
How can I protect her if she won't let me?
The more I think about it...... That's a good question. But, maybe she has a problem with the very idea of you protecting her. Many possible reasons why she might. (Doesn't think she deserves it, for one. Doesn't want to put you at risk and already feels guilty about doing that for another. I'm sure there are more possibilities.) This might be something to talk about. Tell her you feel a responsibility to protect her and are concerned about needing some information to do it well and see where it goes? Be as willing to listen and hear her as you can be. You might learn something and it might help you both. I'm not telling you to try that as much as I am suggesting that you think about it and see if it seems like it might be worth trying.