Not even in his prime could he make 160 a success. He should not muck about with weight this late in his career.
The mix of size and speed at that weight would be too much for Pacquiao to overcome. Sergio lands potshot after potshot, on his way to a late round stoppage due to body shots. Your scenario with the free shot at the beginning of every round is ridiculous. You might as well have wrote Pac gets to kick Sergio in the nuts free and clear every round, same likelihood.