I decided he was guilty without ever finding any real evidence, just based his own words. The man never denied that he was guilty at any point, he basicly just refused to say whether or not he did it, and he also refused to help Saskia live by giving blood. So he doesn't deny that he's guilty, and he refuses to help cure her. The man's guilty. If he isn't, he sure did a great job making himself look guilty. In regard to Philippa, I seem to recall a conversation in Chapter 3 where Geralt can ask her what she would've done if Saskia hadn't been poisoned and she hadn't had this golden opportunity to put the spell on her, and she said something like she would've eventually figured something out. There was nothing about her having been the one to do it, no 'why did you do it', no 'this is why I did it', etc. and this was in chapter 3, well after Geralt would've found the stuff from chapter 2 if in fact there were something that implicated her. Maybe she was going to be the guilty one, then it got changed to Stennis, who knows. But during my playthrough he sure seemed guilty, and I never saw or heard anything to suggest otherwise, that's all I know.