Great ideas here.
Has anyone run it where Venomfang is the real spider at the center of the web? Green dragons cherish manipulating others and this one could easily be pulling the strings here. It's an easy change to make, and it makes the dragon a much richer character in the story.
Venomfang wouldn't want to do the hard work of finding and working the lost mine, but having a greedy drow and a corrupt wizard do it for while some foolish dragon cultists flatter and entertain it seems very fitting. Venomfang could easily be the drow's source of information about the mine.
Venomfang's demands of the dragon cultists could include music, gardening to Venomfang's tastes, preparing fine meals, and general praise and flattery. Not to mention being errand runners between Thundertree and Phandalin, who keep the dragon's identity secret from all but the drow.
The blights are explained easily enough by the green dragon's corruption of the forest. They are part of the garden it is shaping around its lair. Perhaps the blights form topiary shapes for the dragon's entertainment.