'Reduces' is simply the present tense, while 'reduced' is the past tense. The spell is written in the present tense. This doesn't favour 'event' over 'result' in and of itself.
Actually, it does. With the past tense, it's possible to mean either "at some point, you had hit points of zero transiently" or "your hit points have ended up finalized at zero". With the present tense, it can only refer to the instantaneous state.
The spell does damage. If that damage leaves you with no hit points, it turns you to dust. The damage must be resolved before the dust check, and even though it doesn't specify that order (because it shouldn't need to!) this is just as clear an implication as 'dust=dead'.
I don't think it's "just as clear". The disintegrate language about being brought back from the dead is completely nonsensical unless we assume that you're dead, also there's the thing where "turned completely to dust" is usually not surviveable. The order of operations is not quite so unambiguous. After all, in the absence of wildshape, you don't have to "apply all the damage completely". The moment you determine that you've done enough to reach zero, you're done. You don't care whether or not the excess damage is enough for an instant kill, because the target is turning to dust regardless.
Nor would I. That's because I use my thinky bits when answering questions like this. What does the spell actually do in the game world? If the spell forces you to crash through obstacles and doing so causes damage, then it's 'obvious' that the damage is caused as you crash through the obstacle. If a spell damages you, and has a rider that if the damage kills you or knocks you unconscious then it turns you to dust, then if the damage did not kill you or knock you unconscious then it doesn't turn you to dust!
That would be unambiguously the case if the spell description said "if the damage kills you or knocks you unconscious", but it doesn't. Instead, it says "if the damage reduces you to zero hit points". Being reduced to zero hit points does not necessarily kill you or knock you unconscious!
Hmm. There's an interesting case. Consider the half-orc "relentless endurance" trait. "When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead." So, questions:
1. If disintegrate hits a non-polymorphed half-orc, can they use this feature? Consider "but not killed outright"; is there an intermediate state where they've been reduced to 0 hit points, but not killed outright, before the dusting happens? If so, I think it works.
2. So say you're a half-orc druid, and you use wild shape. Can you use this feature while shape-changed? I would tend to think not.
Or look at, say, the Barbarian feature that lets you make con saves to avoid falling unconscious. Same questions there, I suppose.
No, the damage causes the reversion, and reduces the druid's hit points. The spell doesn't pause. It damages the druid so much that it not only destroys the beast form but it also damages the true form, just like any kind of damage.
That's certainly a plausible interpretation, but the disintegrate spell has a special rule here which isn't really parallel to most of the other rules.
There are many things in the game that remove their own trigger. This is how the game is happy to work, and Wild Shape is the kind of ability where the trigger for reversion (0 hp, which means death/unconsciousness) is removed by the reversion to druid form and the switch to the druid's own hp pool.
I'm not sure 0hp necessarily always means "death/unconsciousness". It certainly didn't in 3.x or Pathfinder, where there were lots of ways to remain conscious at or below 0hp.
If we all ruled sanely then, like Crawford himself, we would rule that the druid is not dust.
You've still presented nothing that makes me think that the ruling is somehow inherently much more sane. I think it's probably a stronger case, given the rest of the 5e structure, but the implication that anyone who doesn't agree with you is mentally unstable is sort of rude, at best.
I also think I see another flaw with your argumentation. Under the usual model I've seen people used, the overall conclusion on a revert-to-natural-form is "you were not dropped". Your model distinguishes between the beast form and the druid (or polymorphed) form much more strongly, such that the beast form is in fact being destroyed by the drop to 0hp.
Consider a power like the great weapon master cleave ability. It seems to me that, if you insist that the beast form was indeed reduced to 0hp, but the druid wasn't, that forcing a druid to revert counts as "reduced a creature to 0 hit points". So you'd get your bonus attack. Which you could presumably take on the now-reverted druid.