Do coin tokens accumulate on squares?
Your question is a little bit confused. Coin tokens do not accumulate on map squares. Coin tokens are used only to track coins gained through technologies. Your economy dial tracks the total of coin tokens on your technologies, plus the current number of coins you control on the map. From p. 26 of the rulebook:


  A player may acquire economic power in the form of coins – either coins printed on building markers and map tiles, or coin tokens placed on certain tech cards (such
  as Code of Laws) as a reward for accomplishing particular tasks.


Coins that are lost because of your opponents' interference are indeed lost until/unless you recapture them:


  If an enemy figure blockades the square containing the coin, it is lost while the enemy figure remains there. Similarly, if a building granting a coin to a player is destroyed, the coin is lost.


By contrast, the coin tokens that you earn on your technology cards are yours forever:


  Coin tokens advance the economy dial when gained and cannot be lost.