I guess we have different interpretations on what is meant by very conclusive proof. To me, it would mean a well performed experiment reproducing whatever is claimed. We are at the end of a long, interesting thread, and for simplicity, let me just state that I do accept as fact that both single reed and double reed bends are possible, depending on how the harmonica is blown. The issue I have concerns only the most common draw bend, made on reeds that have a lower pitched reed in the same chamber. When I took the covers off the harmonica, exposing the reed plates, and ddid such a draw bend, I was able to put my finger over and completely block the higher pitched reed slot, thus making it impossible for that reed to participate in the bend. Experimenting in this way, I could discern no difference in musical tone whether or not the lower pitched reed slot was covered. My own experimenting here thus makes this issue puzzling to me.
In his above post, Pat Missin may offer a clue to my dilemna when he states: "As the bend deepens, the lower pitched reed starts to vibrate as an opening reed, at a higher frequency than its normal pitch." Thus, in my experimenting, I may not have achieved a deep enough bend for the lower pitched reed to respond. If this is the case, then, perhaps enough mean air flow is necessary to pass through the slot in order to bend (open) the tongue of this reed far enough away from its rest position, for the necesssary vibration. Speculating further, such a high mean flow (or Reynolds Number) may be necessary to produce enough vortex activity to initiate vibration. Such vortex induced vibrations are very common in Nature.