Doing my second play through and going for the magic resist bonus so I turned down Triss in the elven bath chamber. After I finished my conversation with her, I'm completely trapped in the room. I've tried using aard on the obvious wall but it doesn't break. What am I doing wrong?

Yup, amazing isn't it? Sadly, being over the age of 14, the bath scene just doesn't do it for me and I'd rather have the magic resist stat then seeing some poorly animated sex scene for the 2nd time.

And to respond to your edit, you gain this: http://witcher.wikia.com/wiki/Resistance_to_magic#Abilities. And I've ran all around the chamber and there is no contextual prompt nor does just blindly using aard on the wall do anything for me.

You can't knock the wall down with Aard.

I don't want to spoil it for you without saying more, but you can't knock it down.

The next scene should trigger, if it's not then try reloading a previous save.

No worries, this is my 2nd play so I know what happens. I guess I'm confused on how I'm supposed to get out of the bath if you turn down Triss.

I just took a saved game posted here after the Kayran and did the 20% resistance to magic. I am not sure what the issue is in your game. After you fall through the roof into the elven bath, you cast Aard once standing close to the wall, you get a special mousepointer where you do not even have to select the spell (like when you help Triss heal the elf, you get an Axii pointer even when you have Quen selected as a spell).

The wall does not collaps and a dialog starts automatically with Triss. You turn her down by not suggesting to take a bath, tell her you do not want to loose her (not sure it matters) and that you can't go because you need to clear your name.

After the conversation you walk to the same spot again, you get the same special pointer to cast Aard and the wall breaks. In the character screen the ability is added. So you do not actually cast a normal Aard, it is a quest triggered cast with a special pointer that happens when you stand in the right spot from the wall.

So I am not sure what is going wrong with your version of the quest to be honest.

I don't know about turning Triss down, but normally Roche destroys that wall in the next scene allowing you to leave.