Since Account wide achievements there is no way of quickly finding out if a toon has completed the quests in that zone.

I now need to run around to all the possible spawns on each character and make my Own Spreadsheet just to find out which toon runs with what friend in what zone.

The Problem with these zones in particular is the quests are "Level Locked" behind their skill line progression, so you can complete your current quest and have nothing in your Quest log to remind you.

Also because you can also run Dailies/Heists/sacraments to level up you don't even need to have done the quests, So I now have Thieves guild Level 12 characters that have not done the main story past unlocking Heists, I have some at Level 8 that have not done any Quen quests, the Thieves guild is a hot mess because the NPC's don't clue you in fully either, At least in the Dark Brotherhood the Speaker calls you Silencer and Kor, Tanek and Hildegard give you clues as where you are.

Now I have a wacky idea to fix it, we need some kind of "Quest Log" that lists where your character is within a zone, something easy that you can look at that says "You have completed Wedding Crashers" or dark revelations or whatever on a specific character., we can only dream.