callofstalker: The servant boy hears Olcan telling Stennis he wants Saskia dead, this should count for evidence. It may be earsay(or whatever it is called in english) and generally earsay shouldn't count as evidence, yes, but with a bit of a research you get solid evidence against Olcan preparing for an attempt on Saskia's life so the servant guy is telling the truth about it unless servant boy overheard Olcan talking to himself aloud about killing Saskia.
Yes, I believe the servant was telling the truth (otherwise what use are Axii and Persuade if people can still lie?), but saying that you want someone dead is not quite the same as saying that you are plotting their death and intend to poison them in the council chamber with the goblet. Besides, if Stennis were behind the plot, why would Olcan need to tell him anything about wanting Saskia dead? At best he is an imperceptive and unwitting pawn in Oclan's plan, and at worst he's an accomplice. But I don't think Geralt would have been willing to hand him to the mob unless there was no doubt that he was in on the plot. Geralt seems to generally dislike siding with unruly mobs.
During my first playthrough I thought that allowing Saskia to deliver judgement on him after her recovery was the perfect idea. A third party that both sides appear to respect would be the perfect arbiter. She can decide whether to condemn, acquit, or exact personal vengeance, and Geralt no longer needs to worry about it. Of course, when I realized that Phillipa can make post-recovery Saskia do and say whatever she wanted that idea pretty much flew out the window. Damn meddling sorceresses.