I really like your concept here. If I understand correctly, there would be a new XP modifier called "Optionals Completed" (or whatever they rename it to) and as a percentage of the optionals in the quest are finished the players gain more XP bonus?
This would work similarly to conquest, ransack and ingenious debilitation bonuses... so quests with more optionals take longer to scale up to 25(30?)% for the full bonus?
It sounds like an interesting way to re-work optional XP to remain relevant while removing some of the less-desirable behaviors the dev team want to eliminate. A far better solution than the current plan.