Re: Bokken - from the spine or not?
Ask you sensei and do what he does. It does not really matter. Most Aikido sword work is done to improve your Aikido not to teach you swordmanship. If you want to learn swordmanship, seek a Iaido and/or kendo dojo. Then you'll find yet another "my art is more pure than everyone else's since ...." with regards to how to yield a sword.
In real life, the one who kills the other guys and survives has done the right thing. Thankfully, this is not so much the case now a days since sword fights are kind of history.