Who tried to kill her anyway? I never did find out for certain.

It was never pointed out specifically. In a cutscene that you see if you saved Stennis from the mob, Geralt calls him a poisoner. But within the game, it's not spelled out. The priest definitely was involved, but did Stennis actually get involved or did he just turn his head away and allow it to be done? Someone in another forum made an argument that Phillipa was ultimately behind it, since it would allow her to make the 'antidote' and thus control her.