if you need to use both hands to feel comfortable playing, just don't switch the left and apc parts to record. Leave them as playback if you are happy with them. You can still pound the old left fingers but it will be just dummy keyboard playing, while the right hand records your new part.
as far as extra rhythms are concerned, you can only record one auto play chord track. so to elaborate the rhythms is a job in the composer where you have 2 drum tracks, or use the easy composer where you can combine 3 sets of drum tracks to the final composer. So prepare your composers with varying levels of complexity, just like the variations, pop them in panel memories, and then change the panel memories during the course of the song.
The alternative is to use the pads, which are well suited to augment the styles since two track. In order to avoid playing rhythmically into the pad, record a style into the sequencer, do a apc to smf convert, then a sequencer to pad copy. Any parts of any style can thus be placed into the pads, and looped to play in addition to the normal style for however many measures you want. Instant rhythmic complexity!