Slider or knob, does not make a different to me. I have just tested the UI again and now I understand the use a bit more. But I think there is still a bug with the key lock behavior. Is keylock locking at the original key? 
(Sometimes yes/sometimes no) 

Your approach It is nice, because it only adds a additional control to the 1.11 use cases.  

We should allow it as well. So it is finally up to the Skin and Mapping developer decide which Layout suits most for his use case.  

so we have (keeped the numbering just for reference): 
1. Classic Setup: Pitch control + Keylock button: rate slider + keylock (switch rate to tempo slider and does not change current pitch)
Useful if no additional knob or slider is available 
2. Harmonic mixing Setup: Two sliders: Pitch slider + Tempo slider + keylock (combines pitch and tempo slider to a rate slider)
3. Classic Setup + Pitch knob: Keylock changes and locks at the pitch of Pitch knob and switches the rate to temp sider. (key lock switches actually to 2.)