And of course Crini - and the SC - is correct. It's not that each piece of evidence needs to be proven beyond a reasonable doubt. It's that each piece of evidence needs to be assessed both individually and alongside all the other evidence, and this holistic assessment must then be used to judge whether all the elements of the crime have been proven beyond a reasonable doubt.
But what the SC and Crini get spectacularly wrong, in my opinion, is that some pieces of evidence are so critical to the proof of the case in overall terms that simply raising doubt about them can make all the difference to whether guilt of the crime itself has been proven beyond a reasonable doubt. Therefore, in some cases, it IS indeed possible to say that if one particular piece of evidence is untrustworthy or unreliable, that on its own provides sufficient reasonable doubt to warrant an acquittal. I think that this was exactly what Hellmann was articulating regarding the forensic "evidence".
I've used Bugliosi's "rope" metaphor before for assessing proof, but it bears repeating again to illustrate the above point. Basically, each piece of evidence can be regarded as a strand in a rope connecting the defendant to the crime. Some pieces might be only peripherally proof of guilt, so in this metaphor they would be represented by very thin strands. Some - such as reliable DNA or fingerprints - might be represented by very thick,strong strands.
Each strand is twisted together to form the rope. Then, at the deliberation stage, the rope is "pulled" to see whether there are sufficient strands to prevent the rope from breaking. If there are many thin strands (i.e. many pieces of weak circumstantial evidence), this may still be enough to construct a sufficiently thick and strong overall rope that will not break. Likewise, if there's even only one or two strands -if they're sufficiently thick and strong, the rope will not break. If the rope doesn't break, the court should convict.
Using this metaphor, I'd say that the case against Knox and Sollecito consisted of several small thin strands and a few potentially thick strands: the DNA evidence, the "mixed blood" evidence, the "break-in" evidence and the bathmat print evidence. I'd therefore say that if these thick strands can be shown to be unreliable and therefore removed from the rope, this in itself will cause the rope to break since the remaining several thin strands simply will not hold when pulled.