Complex sentence: Wilco van Rooijen, who had seen Confortola and McDonell helping the stranded Koreans and their guide from below, believes McDonell did not climb back up the mountain, but rather climbed up to the highest anchor supporting the three stranded men to try to transfer the load.
Split into simpler sentences: Wilco van Rooijen, who had seen Confortola and McDonell helping the stranded Koreans and their guide from below, believes McDonell did not climb back up the mountain. Gerard McDonell must have climbed up to the highest anchor supporting the three stranded men to try to transfer the load.