Only the final value of your DR% is capped between -100 and +50.
Yes, if you had 96%, you would have 50% when not cursed, 46% while drecrep'd, and -4% while amp'd.
It would be a pretty bad build overall if you played with 96% DR tho...
Like everyone said, just kill the witches first.
CB dmg only gets reduced by positive DR%.
But it does not deal more dmg against a target with negative DR%.