What about who can use them, if anyone during the defense stage of combat?
Page 33 states that items and powers can be used by the defender. I want to interpret this as:
Player A is defending. Player A can use a healing potion during their defense phase (because its a free action, anything requiring an action point wouldn't be allowed) to heal before taking damage. But Player B cannot use a healing potion to heal Player A while Player A is defending because Player B is not the defender.