Subsequent to last week's experimentation, I have returned to My #3 Vandoren reeds. My clarinet teacher was able to discern some things that I had not noticed when I switched to a stiffer reed, including increased upper lip tension that was wreaking havoc with the sound.
So the #3½'s are on the shelf for another time.
But subsequent to this Amazing Escape from clarinet Purgatory, I did get to feel My own technique with greater acuity, and have retained the ability to flow more easily between the chalumeau and clarinet registers.
I'd say that's worth the price of a box of reeds.