1) Single wall elbow on top of the wh draft hood requires 6 inch clearance to combustibles - like the pipe insulation.
2) Supposed to be a connector/adapter to transition from single wall to b-vent, such as where that elbow connects to b-vent.
3) Single wall elbow should be screwed to draft hood, and b-vent (except that the above-mentioned connector is needed at b-vent).