Law of Unilateral and Mutual Mistake

You are an avid collector and painter of watercolors. You enjoy visiting all of the local and regional art galleries and, routinely, you purchase work of copies of the masters. One evening, at a local gallery, you make an offer to purchase what you are told is an autographed print of a locally famous watercolor artist. The painting is framed in glare- free argon gas glass. You pay $500.00 for the print and glass. 
The next evening the gallery director calls you and frantically explains that you actually purchased an original work by the artist, not the autographed print you thought you purchased. The original painting, in your possession, is worth at least $10,000.00. The gallery director asks that you return the painting, but also informs you that there exists no more autographed prints to sell to you. When you hesitate to return the painting without receiving at least the autographed print, the gallery director threatens you by stating that if you do not return the painting, she will inform all of the art galleries in the state of your refusal and ask that none of the galleries sell to you in the future. 
Aswer the questions below:
-In this transaction, how does the law of unilateral and mutual mistake apply. 
-What about the notion of fraud? 
-The gallery provided you with an express guarantee of authenticity of the autographed print and frame. Did the gallery provide what it guaranteed? 
-What about the element of duress? 
-Is the gallery director potentially liable to the defense of duress if you agree to rescind the contract and return the painting (and hopefully receive a return of the monies you paid).