Maxperson
Morkus from Orkus
The druid has hit points remaining after the damage has been applied, so no dust.
Sure, if you ignore the fact that the druid hit 0 before reversion and triggered disintegrate's ash effect. If you don't ignore that fact, then as written, the druid is ashed and reverted with hit points. There is no rule that says that all damage has to be applied before that effect triggers. The only rule says that if the damage reduces a target to 0, it is ashed. The damage does in fact reduce a wild shaped druid to 0 before reversion happens, so as written it triggers the ash effect.