1 C major only.

2 again C major only. However you could do say a 12 bar blues sequence with all the original chords as long as you start in C major (or set the transpose start to C major when doing the composer editing), it just means you cannot change left hand once the sequence has started playing. It will repeat the 12 bar sequence transposed from any starting chord on playback while you improvise above the split. At the end of the chord sequence just switch to a normal composer variation and play again below the split.

The Technote Americana pattern disks (originally for the KN3000 many years ago now) had variation 5 such a chord sequence composer, it was called the soloist variation. I also described how to create them in composer and pads in a tutorial in the original KN6000 book.