Hi Anon3.
You make some valid points.
From my own experiences (and i've had big problems with posture - i think i'm lots better now

)it is a help to think of good posture in everyday life, to 'break' bad habits.
The 'stand against the door thing is helpful. Another is to sit in the driving position in the car and put your head back against the head rest. 3 or 5 years ago, this would have felt very unnatural, but now, it's almost commonplace (depends on your head-rest of course - and i haven't changed my car).
One thing - on stretching the lower spine - Where would you say the lower spine starts? I would explain this to a beginner more in terms of rotating the pelvis to 'tuck your bum' under your hips. This will straighten the spine. However, it's not a natural position from which to drive forward. Watch top athletes - they stick out their bums big-time.