I was thinking that perhaps the adventure compendium panel could have a visual update by modifying the way it shows your progress.

Instead of showing the highest difficulty you've beaten the quest, it could show your different beaten difficulties... it could show up like this:

Adventure Name: Level: Patron: Progress:
The Smuggler's Warehouse 2 Solo/Party Coin Lords S N
Information is Key 2 Solo/Party House Phiarlan S N H
The Old Archives 2 Party Silver Flame H E

Showing only a letter of the difficulty (S= Solo, N=Normal, H=Hard, E=Elite) in which it has been completed.
I know it could sound silly, but I love redoing quests on different difficulty settings, sometimes when I am invited to do a quest on elite and afterwards, I want to go with another group on a different difficulty, or just try it out solo. It's a small thing, but it could help keep tabs on which difficulty settings you've beaten some quests.

Of course, let the favor gained be shown on the far right.