In "Can't Buy Me Love" the author describes John's distinctive stance "as if it were Elvis idling in neutral" which is funny yes but VERY perceptive. I'm sorry I've forgotten the author's name, I loaned my copy out. He's a drummer and...wow I have to get that book back don't I?
I think you're quote from "Can't Buy Me Love" is pretty damned close to the truth.
I'm male and I suspect most people who post here are also male (tho I may be wrong). I've asked numerous women from my generation (ancient) about John's stance. The consensus is (to put it politely) that John always looked like he boinking a woman in the standing position. So was that what was meant by "Elvis idling in neutral"? Methinks John was doing it for the womenfolk in the audience, not unlike a female singer doing some hip gyrations on stage. Ever seen Tina Turner handle a mic during a sexy ballad? Lucky mic.
As for guitarists holding their instruments high in that era, that was pretty standard. With the 70's, and what was sometimes called "cock-rock", guitarists began holding their guitars at crotch-level, making the necks of their guitars an extension of their penises. In my opinion, the ultimate result has probably been an increase in carpal tunnel syndrome cases.