I don't have status.

When the search results were displayed for my desired flights/dates/times, I selected Comfort+ (W Fare) on a Delta ticketed, Delta operated domestic flight.

I completed the flight selection process and went to select my seats. At each flight where I booked a C+ fare, no C+ seats were available. There was a message saying "no seats are available in your class of service. We'll try and accommodate you at time of departure." There are regular main cabin and main cabin preferred seats available (including exit row), but they can not be selected because of my C+ fare.

1. Should I take screen shots of the difference in fares so if I'm not able to get a C+ seat, I can be compensated for the difference between C+ and main cabin?
2. I understand overbooking, but why make the main cabin seats unavailable for selection?