That is another UX quandary. A lot of the problems I mentioned in my OP would go away if everyone used the standard two finger twist gesture on the screen to '

That is another UX quandary. A lot of the problems I mentioned in my OP would go away if everyone used the standard two finger twist gesture on the screen to 'turn' it up or down. However, if I try and use thumb and forefinger as normal, I cannot get my thumb especially to stay in skin contact with the screen easily - the nail comes into play, losing the double touch on the screen. Using forefinger and middle finger together works better, but makes twisting past a 180 degree turn very difficult. By far, the better solution is the car audio control I posted elsewhere in this thread, which uses multiple finger PLUS slide gestures - but that UI can take up the whole screen just to manipulate a single parameter.