Yes, if you have to click up, then down, then up again, and down to nail down a "just right" value, those buttons would be more difficult but their operation would be apparent. You might be able to get to a "just right" value if mousedown can be held while the cursor is moved between the buttons but a slider is probably better.
The knob behavior you're describing is essentially an invisible slider. An invisible line is better than an invisible arc but it's still not good. Why not make it visible? A visible slider could appear on mousedown so there's something that corresponds to the gesture.
The knob behavior you're describing is essentially an invisible slider. An invisible line is better than an invisible arc but it's still not good. Why not make it visible? A visible slider could appear on mousedown so there's something that corresponds to the gesture.