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You can talk to any adventurer about their clue voyages (those that have them) at any time they are in port, regardless of whether or not you've rolled a clue voyage for them that day.
You don't need to change your region focus for clue voyages for any reason.
The map tables increase the chances of rolling clue voyages, but if you click them they also hint at which island your adventurer(s) need to visit to find what they are looking for. Though the number of map tables you have matters - if you want a hint every day, you need four map tables (down to one hint every four days if you only have one map table).
If you don't give the adventurer these hints (by talking to them in the bar) they'll just pick their own island to search at random. If it's not the island they needed to search, they'll know not to search it again (so your odds of them finding the clue increase with each successful clue voyage, until it is guaranteed).
So map table hints just let you be more efficient in finding an adventurers clues in fewer total clue voyages. Alternatively, you can ignore all this and just send out clue voyages when you get them - you'll eventually brute force your way through (the adventurer then is doing all the work).
I think it's probably not a solid guaranteed once every 4 days. Instead, each map has 25% chance of giving a clue I suppose.