What are the chances of a failure in the starboard propeller pitch change mechanism causing it to reverse pitch?
I'm not familiar with what the Twin Otter was certified to, but I spent a bit of time working on a high-speed turboprop about 20 years ago. We were told to assume a propeller reversing pitch uncommanded would be catastrophic - in other words we needed to design for a probability of less than 10-9/hr.