So Ernestine had a crazy thought. A very crazy thought indeed.
She was going to "turn herself in so you guys could go free". Oh, and then Bengin Transposition herself with someone who is a much more able fighter. Like Jareth, because he has reach/Caster, or Mal, because his fighting style is similiar to the "evil guy".
The threat to make this work is the event that caused Ernestine to leave in the first place. As no one knows about it, with Tira there, even mentioning it could cause a huge problem with the Nandirly. Ern is going to use that as a bargaining chip. "Let them go, or I tell the truth about that night!" Only more threatening.
Or, we say screw it and kick the Revears butts.