Guilty! But not of the murder of her husband. She was guilty of faking a letter that she wrote, forging her husband’s signature, which she then put in a sealed envelope addressed to herself, folded it, and sent it in an envelope to the woman she had befriended in Peru where she and Cary stayed for their honeymoon. Then the woman put local (Peruvian) stamps on the inner envelope and mailed it back to Marsha in California. Generally, a person in California (and some other states) can be declared dead after being missing for five years. Before that time expired, she executed her scheme. If her husband were declared legally dead, the entire Underwood Cellars Operation would be turned over to Marc, and she would be no more than a simple employee. But she wasn’t simple, was she? Under California law, Marc, by the way, could not have his partner declared legally dead because he would benefit by that ruling. So he went on receiving 40% of the profits of what should rightfully have been all his.
On another theme, Marsha had suspicions that Cary was having an affair, but she wouldn’t have killed her husband for his indiscretions, especially since she didn’t even know, as it turned out, the woman he was having the affair with. And in the same way she didn’t want him declared dead because she got almost nothing from his will (remember, they lost a lot of money because of the vine disease), she wouldn’t want to kill him either. Divorce would have been much more profitable than murder.
But an important clue that clears Marsha Underwood is the fact that she can’t or doesn’t drive a car. Remember, it wasn’t just Cary that disappeared that night, his car did also. Whoever left Cary dead in the cellar also drove his car away.