Looked some more at this...
Mono mode effects just the output side of Mobius. On the input side, it still receives from a stereo pair of channels through the selected port, though only one of those channels need to be connected to a sound source. The guts of Mobius behaves as it would in stereo, it's just that only one channel may have content.
On the output side two things are different. First the two internal stereo channels coming out of the Mobius engine are summed together. If you were only receiving on one input channel, there would be no difference in the sum, but if you were receiving on two channels, they would be merged. This summed content is then sent out to BOTH output channels but can be adjusted with the pan control. Pan works differently here, it does a "true" pan, sending a portion of the summed signal to each of the two outputs.
Even though Mobius always deals with audio as stereo channel pairs, when using the plugin you can choose to just not use one of those. For example, route audio in and out of only the Left pin. What mono mode does differently though is if you had a stereo signal going in, they would be summed and sent out the Left output pin.