Well it sounds to me like you actually nailed it but got the steps confused.
From your description, CANCEL_LOOT_ROLL is the event when a player selects their option correct? Is their roll known at that time?
LOOT_ITEM_ROLL_WON only triggers when a player wins, but that's ok because when a player wins then that's the end of rolling.
Can you clarify those two things + let me know what you're actually looking for in your visual representation?