Here's another question to throw out there...

What about mixing conflicts? 
The most common one: when in the middle of a Physical conflict, you throw an Intimidation attack.  Does that works against your Social or Mental track?

I know all the Spellcasters want it to be Social (we want to cast spells with Mental stress).

It's implied that Discipline is what to roll for defense, but you'd still take Social stress.  But if your Social track represents your reputation, not your ability to withstand direct insults with no one watching.

I feel that the social track is your "Composure" track. It's not just your reputation (consequence), it's your cool, your ability to work with others on a social level. Mental attacks have the consequences of madness and lost memories and other mental afflictions. All three tracks could give you jitters as a consequence.