User blog comment:Eddienam/Chapter 53 mystery/@comment-76.31.149.155-20140109032232/@comment-179280-20140109204511

You know, since Ymir regained her human form after eating Marcel, which would be kind of a brutal way of "exctracting" the Shifting power from a Shifter, it's also possible that Grisha found a more "refined" method of getting it, with the side effect of extracting also the memories of said person. I don't know but, that last part seems kinda crazy, since we didn't see Ymir having Marcel's memories anyways.

And about the prototype, I think that Isayama already discarded that part, because it wasn't thought of enough.