A related reason/explanation, is that (from my experience) the ZX2, WM1x, ZX300, have a TRRS separate ground implementation on the 3.5 mm output. If you plug in a cable/jack wired for 3.5mm TRRS into the 3.5 socket, the DAP will use the extra connection as a separate ground, so you will get pseudo balanced. It is not a differential balanced signal, but a separate ground is implemented for each channel, instead of the ground being common to both channels. The DAP will automatically switch to this separate ground output. So the 3.5mm output is already using the TRRS for this separate ground, so it is not available for use by a wired remote controller. There is zero chance that a wired remote using TRRS can be implemented.