The decision point means that they won't be able to found a completely new town using the found town spots because they won't be able to see them. They will be able to take over a ghost town should there be one available, but they won't have the extra 2 points in construction and the $200 discount on founding a town.
There is also no reason why they cannot become the founder of a town should the original founder give them the position. The whole point of this idea is to limit the number of one-horse towns in the game, especially earlier in the game.
You get offered this quest at level 10, but only if you're not in a town. Come to think of it, there may be a case to give the quest at level 15 instead; at level 10 you are less likely to be in a town; whereas at level 15 you are more likely to be in a town! If you join a town during the quest prior to reaching the decision point, I think the quest should halt once you've finished your current task, and only resume if you leave the town. If you join a town as a result of making the decision, the quest ends - you don't get the option of continuing. If you join a town after task 3, then Henry's task disappears; you'll only see Maya's task.
If you join a town prior to being offered the quest, you won't see it until you leave the town and go into the quest menu while not being in a town.