Not sure why the power graph is like that... Might have something to do with their affinity mechanic.
Fort Commanders off the fort should be destroyed, yes. The way the code is written, they should be destroyed if there is no cultural control for that plot... Which SHOULD be wiped as soon as the fort is removed. I'll look at it.
As for forts maintaining their borders... Intended. Any improvement with the 'bOutsideBorders' tag can do that. Prevents it from being completely useless if foreign culture surrounds it.
The power graph gets wierd when the fort commander turns into mobile fortress (str 10) and then fortifies (str 5).