Is this a traumatic response to the initial experience or something more? After all, being willing to go to prison for your principles underlies much of 'real' activism.

Probably a mix of both.

There are a lot of reasons to not want to be in jail: not wanting to be confined, fear of physical violence... but as a middle-aged person with a family, friends and a job, being in jail for any length of time would just wreck my life. I'd lose my job, my relationship with my family would be strained, and I'd probably lose touch with most of my friends. Rebuilding afterwards would be difficult or even functionally impossible.

If I did have my life burned down around me once, my personality would probably tend towards not wanting it to happen a second time, but I could see another person saying "you can't burn ashes" and leaning into it.

The US government shoved her in isolation for years. Even with immunity why should she help them now?

Because if her previous testimony was not perjury, and the government is looking to probe around it, she is likely withholding exculpatory evidence that, were it in the hands of the government, they would be subject to sanction if they failed to turn over to the ultimate target of the prosecution and which might, simply by existing, result in abandonment of some or all of the charges being pursued.

Of course, if the prior testimony was perjury, the calculus is different.

https://shadowproof.com/2019/03/21/chelsea-manning-believes-...

That's a thing in the past.

If she has perjured herself she certainly has no legal right not to be punished for it. Immunity agreements come in the form "come clean and tell us all". If she didn't, well, that was a stupid decision.

If she did not perjure herself there is no trap to be laid.