On April 6, 1846, Dred and Harriet Scott filed petitions in the Missouri circuit court at St. Louis seeking to establish their right to freedom based on their residence on free soil. Under Missouri law they had a strong case. As Fehrenbacher says, "again and again, the highest curt of the state had ruled that a master who took his slave to reside in a state or territory where slavery was prohibited thereby emancipated him." Unfortunately for the Scotts, their lawyer allowed a technical weakness in his case to sabotage the verdict. He didn't provide a witness that could establish that Mrs. Emerson owned the Scotts, even though all concerned knew that to be the case. Because of that defect the verdict went against the Scotts. They filed a motion for a retrial, but the defendant filed something called a bill of exceptions and the case was sent to the supreme court of Missouri. It was not until March 22, 1852 that the state supreme court handed down a decision. The Scotts still had the facts on their side and probably would have prevailed except that the issue of slavery had become so heated in the nation by 1852 that the proslavery justices of Missouri were not disposed to give the anti-slavery side their way. The court ruled against the Scotts.
This would have been a civil, not a criminal case and so they could have compelled Mrs. Emerson to testify. I don’t quite grok this.