During arrival, the winds were reported from 350 to 360 deg at 20 gusting 25 kts, and while landing, they were from 360 deg at 21 kts. The acft's max demo x-wind capability was 22 kts. The plt reported that during the landing, the power levers were retarded to idle, then to reverse. While entering reverse, the acft swerved to the left, away from the x-wind. The plt then advanced the throttles out of reverse to less than flt idle. At that time, the acft swerved to the right. The power levers were again reversed & brakes were applied. However, the acft veered off the right side of the rwy. It continued into mud, the nose gear collapsed & both props were damaged. The plt did not feather the engs until after both props had stopped turning. The right prop was found in the full feathered position, the left prop had not feathered. Removal of the right prop dome revealed no apparent abnormalities or internal damage. There was evidence that the left prop had gone beyond full reverse on impact. No preimpact failure was found.