Sorry, I misunderstood. I thought you were saying the knife DNA hurts RS because it puts MK's DNA at his house where she had never been; therefore, if that DNA is disallowed, it can only help RS. To me, if the knife DNA is lost, it helps both AK and RS, since both were at the apartment that night and yet we have not a single speck of MK's DNA transferred from the bloody murder scene. And at least we put aside the specter of either or both of them running around town with a foot-long knife in hand. Not absolute proof of innocence, obviously, but better than a claim that puts the victim's DNA on a knife in an apartment she had never visited.
The bra clasp is only slightly more convincing to me than the knife DNA. I think it was invented (perhaps with good intentions) long after the fact to fit the prosecutor's "sex games gone mad" theory. Even if the DNA is retested and shown to belong to RS, how can retesting prove or disprove whether there was contamination?
I see it as evidence that links Amanda and Meredith. The fact that the knife was at Raffaele's apartment does not necessarily implicate Raffaele because Amanda was also at his apartment. Raffaele has also stated that he cannot be sure that Amanda was at his apartment throughout the night.
The prosecution does not have to prove that contamination did not occur. If the defence wants to allege contamination, or that the lab has made serious mistakes, it is up to the defence to prove that this happened. During the trial, defence arguments that the clasp was contaminated during collection were not accepted.