Failing or not failing a quest has nothing to do with optional XP.
Optional XP granted on completion of the optional as it should be. If optional XP needs to be boosted there is no reason not to adjust the XP as it is.
You've explained nothing but, that you want to change the entire game punishing those who may not complete a quest for whichever reason to fix a loophole in a handful of quests that absolutely could be addressed individually without breaking the game.
Tired of these blanket fix/punishing honest players to try and stop exploitation which will just find a different path.