[WARNING! This post will be full of spoilers, especially regarding Iorveth's path of the story.] Rockmyheart: What did Iorveth mean when he said there is no more Saskia? Saskia died?
After it was clear that Vergen had been taken (Kaedweni soldiers everywhere!), she and Philippa fled to Loc Muinne. I think that's what he's referring to; that she's simply taken off.
Rockmyheart: How could the witcher be so important that Geralt alone could turn the tide of the battle against the non humans? Or depending whose side you take that could win the battle for non humans just makes no sense at all.
There are several events that happen differently, if Geralt isn't in Vergen, that piled together may have led to the city's downfall. First of all, Stennis dies in the fog, meaning there's no poisoning of Saskia. If you recall, if you're in Vergen, at the end of the lynching (or non-lynching) of Stennis, that's when Iorveth decides to venture out to rally more of the Scoia'tael to his cause. He comes to this decision when he sees how unstable Saskia's current forces are - both the peasants and the nobles act irrationally, when there's so much at stake - and he also feels safe to leave Saskia to the care of Geralt. If Geralt isn't there - and the lynching never occurs - he probably wouldn't feel comfortable leaving Saskia's side, nor would he come to the conclusion that he needs to rally more elves to join the battle. If you recall, Saskia only wins the battle when Iorveth and a hundred or so more elven archers arrive in the nick of time to turn the tide.
There are more things that may or may not have contributed to a successful defence of Vergen, by Geralt following Iorveth instead of Roche, but I believe the elven reinforcements towards the end of the battle, to be the main reason. Then you're free to speculate how Geralt's (and friends') presence may change things (Stennis dying so quickly must have been a really hard blow against the nobles). Heck, even such a small thing as Dandelion singing his uplifting ballad may have played a part for morale. ;)
Ostrowiak: Really, what the hell! He should reply the same!
I think it has a lot to do with pride, the manner in which he speaks to Geralt when you save him during the battle. If Geralt didn't betray him in the ruins by punching him on the throat, and if Iorveth wasn't defeating in a duel with Roche, he might be more honest and upfront with his feelings; i.e. he's not afraid to admit to Geralt that he feels that his cause has been lost (you can really hear the pain in his voice). If Geralt on the other hand betrayed him, and Roche beat him in a duel, he will feel incredibly weakened - his pride severely wounded - in which case he will put on a façade of strength and determination, an air of false confidence, simply to not show his weakness to Geralt. If Geralt never betrayed him, and if he managed to best Roche in a duel, he might feel more comfortable being completely honest with Geralt when they meet during the battle. He outright admits that he himself believes that everything is lost. At least, that's my theory, and I think it's a great attention to detail the writers put into this relationship - how it varies, even if you didn't choose their path for Chapter 2.