Sorry I don't know anything about cord insertion.
If you are worried the doc is being is too positive, you could always ask more specific questions at your next appointment. Maybe come prepared with a list of questions like "Does this condition increase my or baby's risks in any way? If so, how? How frequently does this occur in normal healthy pregnancies," etc.
I did find an old ask the expert response (to a question regarding pre-e & infant loss) where the expert said they weren't aware of any connection between pre-e and marginal cord insertion. http://www.preeclampsia.org/forum/viewt ... 117&p=7706
Good luck, please keep us posted if you learn anything else at your next appointment.