Would their trademark have been the full name spelled out or is it literally "N.W.A.?" I'd imagine that would have previously made a big difference.
Yes, if N.W.A had been used as a precedent to support the Slants it would matter whether or not N.W.A. was written out. The more fundamental question of whether slurs should be protected is separate from the precedential question of whether slurs have been protected in the past.