After the expiration of such six months, the validity of the will can only be contested by infants and persons of unsound mind who had no legal guardian at the time the will was admitted to probate, or who were not represented by a guardian ad litem, who are allowed 12 months after the appointment of a guardian, or, if none be appointed, 12 months from the termination of their respective disabilities in which to contest such will, but in no case to exceed 20 years from the time the will was admitted to probate; and also provided there has not been one contest instituted and prosecuted to final judgment in the circuit court as is provided for in sections 43-8-199 and 43-8-200; in which case the final judgment of the circuit court, court of civil appeals or supreme court shall be final and conclusive against all parties.