Almost certainly blocked by NDA; that is her inviting Congress to issue a subpoena so she has legal cover.
Not really. If the NDA is void (such as the case with congressional hearings), there isn't going to be a lengthy legal proceeding. The judge would look at it and throw it out.
People brought before a Congressional hearing like this can be held in contempt of court for failure to cooperate. Testimony is subject to perjury laws.
This wasn't just someone writing a letter to Congress with some claims. This was a Senate hearing convened on the matter.
A Congressional hearing can compel people to give testimony under oath. Witnesses can be held in contempt for failure to answer questions.
A corporate NDA cannot prevent you from cooperating with a Congressional hearing.