So you build your city on a server, and it has 6 neighboring cities created by other players. What if those players never return? Do the neighbor cities simply get replaced by new ones?
Pretty much, you can choose to abandon a city...honestly though I doubt that if you really left the game to never come back you would abandon the city.
Also, I find the speed control thing odd. Say you and a friend are playing at the same time. He plays at speed 1 and you play at speed 3 and the region shares resources and what speed do the cities share resources at?