I believe the mp3 encoder in S1 is hardwired to do stereo output. The reason might be that many mp3 codecs out there will actually create a "doubled mono channel" for a "mono file", which is, technically, still stereo.
If my assumption is correct, you may have to add a tool to your pipeline. FFMPEG is free (ffmpeg.org) and you could create a batch or do it manually - something like this:
ffmpeg -i YourExported.wav -ac 1 MonoOutput.mp3
Note that I would not export an mp3 from S1 for this kind of pipeline because you would compress your original material, decompress it in ffmpeg and recompress again, which would most likely lead to lots of artifacts being introduced.
You can use ffmpeg to reduce the bitrate, too. Have a look at its documentation, it's quite extensive.
I hope this helps!