Might I suggest to wait until they are about 4-weeks old and then post some pictures. So many things change from this age until then. I would have to question them being Marans from the simple fact that they came from eggs that were so light in color. Even the poor egg colored Marans are pretty dark in comparison to other breeds, are they not? The second one very much resembles an Australorp. However, if your breeder/seller says they had Black Rocks, then perhaps you have one or more of those. I don't think the one with the heavy striping is a Cuckoo either because the stripes are too well defined and even, more like a Barred Rock than a Cuckoo. Just my observations. Patience will give you more answers than just guesses.
Good luck
Cheers.