I've not actually played with the A to B facilities, although I've read the manual and it looks like you should be able to do what you want

Alternatively, could you turn the intro, verse and end bits into the intro, Variation A, and Ending parts of a user style? Then the "middle 12" would repeat ad-infinitum until you pressed the "ending" button.

I assume you can effectively turn off the "follow the left hand chord" function by setting an accompaniment split point to somewhere "to the left" of the end of the keyboard.