"This has been covered in detail. The above is possible. But what it achieves is to tell the distributors that TWO studios have leaked. And which ones they were. Basically, you think using n sources hides which one of n was the leak. It doesn't, it provides a list of thise n studios that have been compromised."
That depends on how they're forensically identified and how one goes about removing the traces. If they're all "add a frame here and there," the pirates can default to "trim anywhere an extra frame is detected," which would basically whittle down the forensic tagging to the point the studio won't be able to tell which studio got raided. If they're all "cut a frame here and there," you do the reverse and extend with the same results. As for "a mix of cut and trim," if you mix them up, then it's going to be much harder to tell which copy/copies got nicked because you also stand the chance of coming close to colliding with the signature of a THIRD copy, raising the possibility of a false identification. Oh, speaking of third copies, if the pirates obtain a third copy, they can probably defeat the signature reliably by using a "two-out-of-three" rule, keeping the clip length that appears in two of the three copies (and in the event of a three-way-tie between cut, extend, and nothing, keep the nothing).
And then, like I said, there's the time investment required to make each copy forensically unique, since even professional hardware takes time to encode a 1080p video.