[AreaOfEffect]

In regards to the assassination issue I'm sure it is working as designed. When you siege a castle you technically own the province. This means that all random events go to the sieging nation. The sieging nation gets to control the tax rate and they also have the ability to declare prophets. All of these are abilities that only work when the province is yours.

Assassination is designed to only work when in a province controlled by the enemy. I'm sure the ownership of the province is the flag that decides if the ability is active. It would logically be a pain to program assassination for the sieging faction as this is a very special case.