The NFL stated, in the suspension letter that the suspension WAS NOT based on Thompson's allegations. Therefore, she is not the main witness. It's BS, but it's BS that negates the point you are making:
As the arbitrator, Goodall is given the power to decide who can/can't be called as a witness, as the 2nd circuit confirmed last year:
So, the 2nd has said Goodell has free reign to decide what testimony he wants to include and exclude. That would likely include the testimony of Thompson, and of the person who interviewed her, as Goodell placed more importance (& based the suspension on) the pictures, medical testimony, & other evidence. Just because the NFLPA says the suspension was based on Thompson's testimony doesn't change the fact that the NFL explicitly said it was not.