Had both of these points happen to me before, these seem to be the generally accepted ideas with them.
1) Any model that makes their attacks on Obyron himself and miss will add to the bonus Cleaving Counterblow attacks, any models that just attack his unit generally will not.
2) Necrons that have Gauss weapons and the Tank Hunters rule thanks to Zandrekh do get to re-roll any of their armour penetration rolls. This is simply because all the Gauss rule does is give them a chance for glancing against any vehicles, even if their strength isn't enough in normal conditions and the Tank Hunters rule just allows them to re-roll all armour penetration rolls.