Clockless CPUs with RF transceiver and signal manipulation logic, however, do not exist.

grain-sized? and they still need power.

Honestly, I'm too ignorant to judge the rest, I just think you weren't accounting for (1) you don't need a clock signal for fancy logic and (2) you can fit a lot of logic in a cubic mm. The other points may rule this out, I don't know. Note that the async CPUs I've read about tend to be lower power than clocked ones.