Ok, I think I understand everything now. In summary...
1. The Volpe can accomodate up to ~38mm wide tires.
2. The Volpe can handle 35 mm wide fenders, and 45 mm wide fenders are definitely not a possibility
3. With some manipulation, the SKS 35 mm fenders should be accomodate 32 mm wide tires.
From what I've been reading, I should be able to use 32 mm wide tires on dirt trails (I weigh 170 lbs.)