I am a priest.
Gustapho won't speak to me, he keeps saying he's busy so I can't speak with Kathryn. I have both the amulet and the book and I've helped proven Corin innocent whereas she confessed. So I took another approach and decided to go after Christoffer De Malamert, problem is, he's nowhere to be found. I've searched every place on all the possible places (even the citadel) but he's nowhere. So now I'm stuck without being able to continue.

When I first arrived to Alvaron I decided to investigate on my own and I got so far to get Downay convicted along with all the rest of the minor heretics (except Sven). I also freed Corin. I have the amulet of death and the book but I can't perform the ritual. Malamert is nowhere to be found when I decided to help De Guise to convict him.
What should I do?

Have you been in the citadel's dungeon ? Malamert is at its very end.

To my knowledge there are 3 floors of the citadel and I've yet to find the dungeon. Where is the entrance to the dungeon? I could only find the stairs which leads to floor 2.

The map looks glitched, not sure if it really is though.

http://i1207.photobucket.com/albums/bb467/DisparagingSubjection/Inquisitor2012-12-0515-20-05-183.jpg

http://i1207.photobucket.com/albums/bb467/DisparagingSubjection/Inquisitor2012-12-0515-23-09-130.png

http://i1207.photobucket.com/albums/bb467/DisparagingSubjection/Inquisitor2012-12-0515-19-50-652.jpg

The entrance is on floor 2. It looks like a kind of elevator (you can see it on the left of your third screenshot).

Edit: Seems I was really tired when I wrote this as I mixed up left and right...

Thank heavens I found it. Now in hindsight I feel like a complete idiot haha.
I thank you a thousandfold!