My MK2A does double whatever single channel is attached to the AUX IN line, but still maintains proper stereo with both L/R attached. What exactly is the problem with it doing that anyway?

I believe this is done internally by the SAA7705 DSP. From looking at the DSP datasheets, it appears the DSP enters what is called "Speech Mode." Whenever it encounters a mono signal, it delivers it to both channels.

From page 19 of the SAA7705 Datasheet:

The speech mode offers the function that a mono, e.g. a speech signal, can be applied at one channel and reproduced by all output channels. The preferred input is the AM_L input but it is also possible to use one of the analog CD or Tape inputs if desired.

Stu