Glad I could help you out. I have a working theory that C3D tends to not accept the polyline if it was created before the surface was. Even if it appears to be spot on.
Totally get your logic on how unaltered existing is technically part of proposed, but my preferred workflow is to keep proposed as just the design changes. I like to keep it as simple as possible. You'll figure out your best workflow as you go along.
Normal people believe that if it ain't broke, don't fix it.
Software Engineers believe that if it ain't broke, it doesn't have enough features yet.