I have the chamberman's testimony, and the scratchmarks on king's face but apparently that is not enough to accuse him. Who else do I need to talk with?

This is a power example - perhaps, the more emblematic - of the awkwardness of the design in this game.
You must solve Trisstan's quest before you can accuse Rodrigo.

You need the testimony from Cardinal Gustapho Letti in NE Alvaron. However, he will not talk to you until you have the letter for him from Grandmaster Cristoffer de Malamert. The grandmaster can be found at lvl 4 of the Citadel dungeons.