If the stall speed increases in a steep bank because of the reduced effective span
Stall speed doesn't increase in a steep bank due to reduced effective span. It increases because of increased G-loading, thus increased angle of attack.
Slips do more towards reducing airspeed by putting the fuselage into the airflow than anything about span-wise flow. The increased rate of decent is a result of putting the nose down to maintain airspeed given the increased drag.