I understand. Unfortunately, GW will not be able to fulfill this attempt. Correct me if I'm wrong, but this is their first card game. Although I'm positively surprised how well they did it, there will be a constant "greyness". Thus, a judge is needed during tournament in your scenario of "sportsmanship doubt". During a friendly game you can look from a "tight/competitive" point of view: we all have played many games and re-roll is just repeating the roll, if you do it later, then it's not re-rolling anymore, it's modifying the dice result. For example, the ability of changing the result in this game would be in a form of "reaction: after your opponent rolled the defence dice, you can re-roll the attak dice and recount your number of successes." I just think this is a problem category when opponent tries to bend the everimperfect rules, I might be wrong though.
If you focus only on the time part of the Gurzag's ability, then what stops you (besides common sense) from re-rolling the defence dice?