Finally built the line input box for the recorder. It was just 10uF 50V blocking caps connected in series with the line level. The computer output was referenced to ground & the ADC input was referenced to 2.5V. The computer output the 5V peak to peak voltage required to reach the recorder's full dynamic range.
The recorder simultaneously handled the balanced microphone input & mixed the 2 inputs into the headphones, as designed.
This feature was completely forgotten. The original idea was to record I2S directly from the computer's optical output, to give purely digital copies of whatever it was playing. The problem was the sample rate of the I2C & the microphone inputs would never match & the I2S signal would stop when the computer stopped playback or changed sample rates.
A better way to record computer audio was believed to be pulseaudio, jack, Esound, or ALSA, but no sound driver was reliable enough to stay synchronized with the microphone track. The final solution was recording the analog line output with the same clock signal as the microphone, as was already implemented.
The audio recorder still doesn't have the jumper for wifi adjustment that the vlogging microphone had. The wifi has to be manually increased to 30dB in order to transfer a file & 5dB in order to record.