I had thought about this issue when I first wrote the game rules. Moving a stealthy army into enemy territory is not against any explicit rule as long as they do not actually attack a controlled province. Keep in mind that discovering a stealthy army is really an attack by the party which owns the province, not the stealthy party.
If a sleathy army gets caught and a battle ensues, that I believe it would fall on the involved parties to resolve it as best they can. Most likely, there would be a Council proposal (or even 2 proposals!) on how to handle the situation if the stealthy army were to win the battle. Surely a group of intelligent wyrms could decide such a wee issue, right?
With the right bribe to Ronan the Mighty Centaur-ruling Wyrm of Pangaea, I think ANY issue could be easily resolved in Council
