How very odd. So many chickens start out like little chipmunks so they are hard to peg at that age if you don't have an idea of what they had. Have you reached out to the 'breeder' to ask them what their thoughts are? Perhaps they are more scrupulous than we think and would offer fertile eggs...
Those are definitely not Australorps. You mentioned the breeder had Marans, but you are unsure of what variety. Do either of the chicks have lightly feathered feet? They'll just appear fuzzy at this age.