Finally conceded defeat & spaced the boards out again, as far as possible in the arm band. It was an acceptable compromise for the quality sought after. Slowed down SPI to compensate for the longer cables. The 2 minute transients were gone, although wifi txpower still had to be 5 or below. Soldered in the headset & reenabled the DAC for the 1st time since enabling SPI. SPI immediately started glitching.
3.3V required a big cap to keep SPI from glitching, with the DAC. With the headset being amplified & monitored, the SNR was probably obliterated, but at least it would be the lowest possible. The monitored sound had lots of wifi interference, because the DAC is powered directly by the PI's 3.3V. Surprisingly, it had none of the aliasing which was experienced on day 1.
The quality of the recorded waveform was ludicrous. There was no interference & the fixed gain isolated voice from background noise far more than the phone. The background noise could be amplified to 100% without noticing any interference.