The worldmap can hide quests that are completed by your warband already, we should do the same for Wholly.
We also could show which characters on the same wow account (not bnet) have done the quests in the tooltip if we had logged in with grail on that account.
For everything else we could rely on IsQuestFlaggedCompletedOnAccount

I am not sure I want to start adding the ability to record a character's completed quests into the main Grail.lua database file as it would get much larger much more quickly. This is the only real way to be able to know which specific characters completed the quests. However, I just pushed a change to Wholly to allow it to filter on warband, so you not show quests that have been done by the warband.