I don't find Knox's identification of Patrick Lumumba as the killer to be exceptionally damaging. Knox had been interrogated by the police for 36 hours straight at the point she named Lumumba. I find Knox's story that the identification of Lumumba was coerced by the police believable.
To me, the more damaging story was the initial story that she spent the entire night (for the first time) at her boyfriend's place. That story was disproved by phone and text records and her boyfriend's testimony upon his own interrogation by the police.
Yes, exactly, maybe changing her alibi once can be explained by stress, pressure, trauma, etc. But, more than once makes her look like a liar.