Notes re the stain on the pillow:
It would, presumably, be possible to approximately date the stain (semen or otherwise) on Meredith's pillow - but only if the pillow had a pillowcase over it and if it was the pillowcase that the stain was on. My following argument is based on the assumption that the pillow DID have a pillowcase on it - so if anyone knows otherwise, then my argument becomes fairly null (and in that case I would apologise in advance for wasting everyone's time!).
It might have been possible (in November/December 2007, at least) to establish the following things: a) that the pillow was from Meredith's room; b) that it was used regularly by Meredith as bedding; c) that the pillowcase was either Meredith's own or was provided for her use, and; d) the frequency that Meredith washed her bedding (even perhaps, via flatmate testimony, the actual last time that Meredith changed her bedding).
If all those things could have been established, then the date of the stain on the pillow could have been narrowed down. For example, a wild guess would suggest that Meredith might have changed her bedding once every two weeks at a bare minimum - so a worst-case scenario (that she was on the verge of changing her bedding on 1st November) might suggest that the stain was a maximum of two weeks old.
Then, if the stain HAD tested positive for semen, and a DNA profile could have been extracted, investigators might be able to conclude that whoever had deposited that semen had done so within the previous two weeks. If the semen DNA was identified as Meredith's new boyfriend's, it would obviously be of no evidential value. If it was identified as Raffaele's or Rudy's, this would be powerful evidence against either of them respectively. And if it was identified as belonging to some other male, police would have had to try to discover whether Meredith had had consensual sexual relations with any other men in the two weeks before she died (unlikely, I'd suggest), or - barring that - there was another male involved in the sex/murder crime.
Obviously, all of this is total supposition. The stain might have been Meredith's saliva, food residue, hair product - many things, I'm guessing (although the scene-of-crime investigators seemed to be confident that it WAS semen). I also understand that there was an issue of damaging the other evidence on the pillow, if the "semen" stain had been swabbed for analysis.
Nonetheless, I can't help thinking that the police overlooked a potentially very useful piece of evidence here. It MIGHT have been useless (e.g. if the stain was semen from Meredith's boyfriend). But it MIGHT have provided important clues as to who committed the crime. The point is: the police could not tell, on 2nd November 2007, which of these two possibilities was the more likely. So they should have tested the stain to find out.
One last thing: I think that any testing done now (even if it could be done now) would be of slightly less evidential value. This is because I imagine it would be harder to establish a date range for the stain after so much time has elapsed (by this, I mean that the other girls in the flat would probably have forgotten anything they might ever have known about Meredith's laundry habits). But I'd argue that if the stain came back positive for semen, and that semen matched Raffaele, then this would be very damaging for him. I appreciate that some think Raffaele's defence team is playing a very sophisticated game in requesting testing at the 11th hour, but I'd be more inclined to put that down to defence apathy and/or incompetence. After all, playing that game would be an INCREDIBLY high-risk strategy, if Raffaele's defence team actually thought there was a real possibility of his semen DNA being on the pillow.
Of course, if the stain turned out to be Guede's semen DNA, then this would be further evidence against him (but I'd argue that it wouldn't really change things for AK or RS). However, if the stain were found to be semen from someone other than RS or RG, then things might get very interesting......
