I vaguely remember TA from eb talking about issues with disasters and the solution he was proposing at one time was to delete the map.rwm file and let it regenerate. I don`t remember what the consequences of not doing that were but not being able to construct a fort/watchtower might have been among them. So, are you sure that what did the trick in your case wasn`t just deleting the map.rwm file with no actual effect from the change in descr_mercenaries?
BTW, this thread should get a better position than under the sub-forums(maybe it should be placed above them). If you don`t scroll down you don`t notice it and I see that in 3 years it only got 11,016 views and not many posts, when in fact it is one of the first places a modder should start from.