If it's a touch screen you just drag the channels around, no vertical knobs needed.
Not if the trace is off the screen!
You're just not imagining.
If it's off screen you can have a marker arrow at top/bottom to show where it is. Touch the arrow to grab the trace and bring it back on screen.
And you still need a "vertical knob" for vertical resolution control. Controlling that with pinch gestures is very imprecise.
Says who? If it's done well and snaps to the usual voltage ranges then it'll work perfectly.
Want fine control? Double-tap it to bring up a numeric keypad and circular control knob.