My MiL was diagnosed because she is also manic depressive. They nailed the narcissism when they were evaluating her for meds for bipolar disorder.
In terms of your question: "if you believed she felt the same, what did you attribute her lack of interest to?"
Ha! I couldn't figure it out at all. It tortured me for years! At one point I was about 80% convinced she must be gay. Ironically because she seemed to have no interest in sex at all I also figured she'd never have an affair -- what would be the point?
I know better now!
Married 18, Together 20, Now Divorced M: 48, W: 50, D: 18, S: 16, D: 12 Bomb Dropped (EA, D): 7/13/11 Start Reconcile: 8/15/11 Bomb Dropped (EA, D): 5/1/2014 (Divorced) In a New Relationship: 3/2015