Yes and quartzites are, by definition, full of this silica which...

Yes and quartzites are, by definition, full of this silica which fills and blocks all porosity, both intergranular and fractures. I suspect the Heavitree isn't a true quartzite in the metamorphic sense (which requires full recrystallisation of the grains) but rather a quartzose sandstone with near total cementation of porosity with diagenetic silica, but in reservoir terms the difference is pretty academic.

The only way there will be fracture porosity here at Dukas is if a fracture network has developed after the rock has already been turned into quartzite, and there hasn't been another major burial period or introduction of hot fluids after that.

It is possible and does happen but the chance is fairly low. I don't know enough about the burial history of the Amadeus to know if it's feasible, it would depend on the timing of burial and compressional or extensional events. I know there have been several of each since the time of Heavitree deposition so... maybe!