The FTP settings should have no impact on a virtual route, only in a planed training ride where you target zones. Some trainers have a max gradient, and usually those steep sections are just short anyways I wouldn't worry about it stating 13% it's likely a very short portion. You're right about the bike weight it makes a difference. One thing about zwift if you're not already doing it is setting the difficulty to 100% in order to get a proper comparison.