The big mistake they made with the marksman was that all the spells were instantaneous because he can no longer make combinations like the rogue

This could be easily resolved with one of the suggestions I made: reducing the cooldown of the Spread Shot ability from 3 to 2 seconds and slightly increasing its damage. This would allow for a continuous rotation between Hellish Volley > Spread Shot, similar to how mages without many area spells usually hunt with blockers or in AoE (Shaman: Hailstorm > Whirlwind) and (Warlock: Cross of Annihilation > Whirlwind).