1) One insert is for upper lower parts. One insert is for auto accompaniment.  Upper 1 usually takes priority, or the first chosen tone with an insert effect.  All other tones in the lower and other upper will not have an effect.  You can manually reroute DSP line 2 to another upper or lower part using the mixer, but keep in mind that will leave rhythms with no insert effect option. 2) See answer 1 3) Each layer has a DSP on/off button 4) No