I see your point on the registration button, but if you have both the current style and the previous style directly in front of you, (Up to 9 in the case of the Abacus) then you can change back to the previous style (Just one button push on the screen) without changing anything else.
Also with 2 button presses onscreen you have direct access to 45 more styles which can be selected without changing anything else. (Just 2 presses to get back to your previous style)
I do however see how a back (Jump) button would be very convenient, and I will suggest it to Wersi, as if enough owners also suggest it I am sure Wersi will incorporate it in one of the regular updates. (A lot of new features or modifications (Many are just small ones) etc in the updates have usually been suggested by Wersi owners)
Enjoy whatever you play

Bill