Re:Forgettable, though Boney's previously said that the specifics of the common Marks vary from person to person, he's also said that Forgettable relates to personal identity:
So managing its effects likely involves mentally juggling your own self-perception. Mathilde could, for instance, force herself to briefly believe that her identity is centered around her hands or feet, so that people couldn't remember them rather than her face.
She might also be able to do the opposite: make herself temporarily believe that her identity is all of her, making her very hard to remember when she's trying to pass unnoticed and unremarkable somewhere.
...I wonder what part of Mathilde's soul is tied to Flicker or Mantle of Mist. Being intimidating? Being confused?
I also wonder: can we even control Shrouded? It doesn't seem to have any downsides to it, and I'm not sure it can be improved upon.