Is Barton's right under the instrument valid?
Yes, because Barton's right to purchase will vest or fail within the period prescribed by the Rule Against Perpetuities. 
No, because Barton's right to purchase is a restraint on the owner's power to make a testamentary disposition. 
No, because Barton's right to purchase is an unreasonable restraint on alienation
Yes, because the instrument is recorded. 