Just read the manual of DJM-800. It has a Harmonic effect knop, like you suggested. 

After some additional thoughts and investigations: 1. still suites best to me. You have full control with a single slider and a button. The additional knob is IMHO overdone, but we should allow it anyway. 

By the Way: What about our master sync branch? Will we have a "Master Tempo Lock" CO? What happens if it is activated with the Pitch slider and the pitch of the track?