Sounds like something is dragging enough to allow the engine to crank but not fire. With the belt disconnected have you checked that the pulleys/tensioners that the serpentine belt run on are free to turn by hand?
I can see from a diagram that the belt drives an aircon pump, power steering, alternator, water pump, idler pulley and tensioner. Any one of these could be seized/ difficult to turn causing slow crank speeds or failure to crank when the belt is attached. Hope that helps