Bob, the original Poly-800 uses six C batteries primarily as backup to preserve the internal memory, although the batteries can be used for general operation in lieu of the AC adapter, for use in performing as a "keytar." Evidently the previous owner modified the unit to use a CR2032 for backup purposes in place of the C batteries. As long as you have an appropriate AC adapter, you would not need both the C batteries and the lithium button battery.
Regarding the audio interface for loading sounds, I have also tried unsucessfully to save/load sounds to/from my laptop and mp3 player, and thus need to rely on my old cassette recorder. I believe the problem is because the Poly-800 interface was designed for analog signal and therefore does not correctly interpret digital signal. Someone else in the group may have better information.
Greg