If you made sure that all connections of the cables both at the plug you stick into the Pi's jack and at the socket / jack at the other end of the cable (where the earphone is plugged in) are properly isolated, there should be no issues in leaving the cable in place all the time.
Note that the sleeve of the jack is connected to the video signal (see here). ...
I was using an incorrect route.
The RaspberryPi has no audio input. It is not possible to configure the jack for input. It is OUTPUT only.
The correct route is:
MacBook AudioJack -> TRRS cable -> MicIn-of-USB-Soundcard -> Rπ USB
Now my Rπ is able to decode the signal.
The problem of intermittency experienced by one ...
For anybody interested in the solution to this problem when two devices have the same snd_ identifier:
In alsa-base.conf, use the vid and pid attributes on options (they both accept comma-delimited array items, generally 0x1234 format)
You can get the vendor ID and Product ID of your device using lsusb, then set the device order normally in alsa-base.conf