Hello John
Yep, there sure are a lot of interpretations of every pose!
It is definitely much better and safer to keep the back totally straight through this breathing exercise. It is really only the neck that bends. Keeping the knees locked especially hard on the exhale will guard against that tendency to lean back or push the hips forward.
One little tip I would like to offer: when you inhale you are loosening up everything trying to get very tall and long. If your legs needs a little break from being completely locked out to just being firmly engaged, then on the inhale is the time to do that. It is extremely difficult to keep 100% leg lock for the entire Pranayama exercise. Engaged and firm is possible.
Just before you exhale, tuck your tailbone under to lengthen your lower back and make your entire back straight, lock your legs as hard as you can and complete your breath cycle. There is a straight line from the back of your heels through your strong legs and back right up to the bend in your neck.
As you mention, shoulder relaxation is also key. Thanks for your question.
Namaste
Gabrielle