I think you've covered some interesting angles on the "douche" phenomenon. The only thing I'd add is that it's fun to say. I think this has fanned the flames of its popularity.

Not to mention the imagery conjured by those old Summer's Eve ads.

It's also a bit of onomatopoeia.

Douche, or "Dooosh" is the metaphysical sound an annoying person makes in conversation. It's the sound I hear in my head when Glenn Beck speaks for example.

I also am curious how the word douche and it's rise to popularity maps to the same for other popular or similar labels used in the past, like "square" or "roustabout" or "pain in the ass" or "wonk" or "diva" or "dilettante".