Can I make an ultrasonic motion sensor using one piezotransducer?
At 33kHz, the emitted wavelength is:


  340.29ms^1 / 33,000Hz ~= 0.01m, i.e. 1cm


"Nearest distance is 0.4in (1cm)"
The wavelength of one pulse is the same order magnitude as the detection distance.

The ultrasonic sensors I have seen, where I found adequate data, seem to take 2-3 cycles to get to full oscillation, and a similar number of cycles to stop (IIRC a little longer to stop than start). IIRC the active pulse duration was several cycles, so 8-11 cycles in total. 

I can't find the link, and I can't remember if the 'stop emitting' was being actively driven, or it was just switching off the power. So you might be able to do a little better by driving the device more effectively.

Purely based on that, I'd expect the minimum detection distance would need to be slightly longer than the length of the pulses round trip journey to make it straightforward to do.

You might be able to characterise the system in such a way that it can detect reflected sound during the 'stop' phase, maybe halving the minimum distance. It might even be better to emit for longer periods, though you might then be unlucky in the interaction of emitter and room reflections. 

I believe some bats emit at a wide range of frequencies to help detect target type and distance.

If the 1cm minimum detection distance is a key requirement, maybe look at higher frequency devices or multiple devices. The higher frequency emitters I looked at had much narrower 'beam angles' than lower frequency devices though. So that might introduce a different constraint.