Why not go to the jury, repeat previous statements, answer any question she's comfortable with and go with the tried-and-tested "I don't recall" for everything else? Or plead self incrimination if there is a risk?
This must have occurred to everyone involved. There isn't any reason to go to risk jail rather than answer a few questions.
Heroes don't play dumb, heroes do the right thing, make a statement, send the message ... and she is a hero, not someone trying to cover her ass.
On the other hand, whatever she do, she's screwed, her future in such legal and power system is not bright anyways. So why not trying to be "die as a hero" then, it's not likely she would "survive" without "selling out" anyways?
She's decided to not play that game. To not serve as an accomplice in the US Government's case against Assange.
I don't know why you think there's nothing heroic about remaining silent under intense government intimidation. You can come up with a counterexample very trivially -- in the civil rights movement, if you had witnessed a black person going into a "whites-only" bathroom then it would've been far more courageous to refuse to act as a witness than it would be to tell the truth.
There is also a long history of resistance to grand juries among anarchist activists, as a policy of refusing to give evidence to the government and law enforcement organizations: http://grandjuryresistance.org/ https://en.wikipedia.org/wiki/Seattle_grand_jury_resisters https://earthfirstjournal.org/prisoners/grand-jury-resistanc...
Because of being a principled person and doing the right thing? If anything Manning so far has shown to be pretty consistent in that respect.