1.) You can temporarily block people and unblock them later.. Alternatively, you can use a 'fence gate' furniture (or a keyblock) and use the permissions (or a password) to filter who can come in or not.

2.) A blanket option (lock or unlock all) would be nice. The current individual gralat-locking does get a bit tedious!

3.) What would be the use of this?

4.) This would be cool, but we might not see it for a long time because it would be difficult to implement. it has been brought up before - specifically in relation to 'guild houses' where you could have your guild members be able to build/move furniture stuff.