Enter could be. I think you are complicating things here. You can always set the default attitudes for a structure to cooperative if you want anyone to be able to enter or leave it. Stacking needs to be hierarchical. In other words, as things are today if you stack with somebody you have to trust him or her totally as he execute you at will without risk.
Hierarchical stacks prevent that.
Well, population figures for buildings are not all that elegant, but IMHO anybody should be able to see whether a castle is an empty hull or hundreds of people live inside. That those not require observation skill. If you attack something owned, you attack the owner.