To answer my own question about mono input: I just checked the schematics, and the headset plug is indeed connected as stereo out and mono in. The respective input is LINPUT1.
The audio chip does support stereo input though, and the connections on the mainboard go to the (internal) connector right below the headset connector, labelled “Line In” or “J16”. Those are connected to LINPUT2 and RINPUT2.
This brings me a bit closer to understanding how to get audio-input working, but there’s still the question what to set in alsamixer to actually get it working…
I’m planning to skim through the audio chip datasheet, to find out which flags need to be set in order to have working input from either a microphone on LINPUT1, or a stereo line-in signal on LINPUT2/RINPUT2.