Do you maybe have the latest version of MixPad? Because on mine, on the laptop, if you over it, it does nothing. You really have to press on it and then slide right or left, with both, but when you do that, obviously, even if you move ever so slightly you have already moved away from wherever it was. It does not seem right. So basically you never really know where it was and have to trust that when you open a new project, it is on 0 and never touch it.
As far as the Mono for recording vocals, why, if you have it set on Stereo, records mono?
Maybe you meant that when you hover above you can see the values, not move the slider? You can do that only with the volume slider, not the Pan left and right, not in mine. What is you see is: Stereo Left Righ Pan Playback Volume.