I don't know, but the imagery described depicting Mary in a pose akin to that of Ceres sounds interesting - to me at least. It would be good to actually see it. As far as 'continuance' is concerned, I don't think anyone's asserting that Mary is secretly Ceres. Just that she became associated to the same festival period, religious role and agricultural imagery. It's not a great leap to suppose that it may have been a deliberate move on the part of the Church to ensure Ceres' agricultural and religious functions, was imitated and adopted by Mary.