1) The Devs HATE Hearthvaults! They hate them to the point of wiping the area of a hearthvault and killing characters on an account.
viewtopic.php?f=6&t=17564jorb wrote:Existing hearthvaults will be salted with plutonium, and I will personally summon and kill anyone found residing in one. Get out of them or die. Send me a hearth secret in the vicinty, or the account and character name of a character camped in the vicinty, and I will come over and, in short, nuke the place from orbit if a ram cannot be successfully placed.
That's why you leave a 2x2 area for a ram.
2) Essentially they can hop your walls by exploiting Village charter stones outside of your city/village/hermitage/whatever. By having a village claim you make it more difficult for them to have a village claim near you and place a charter stone. By placing unfinished constructions and junk in the way of landing, the charter stone will repath you until it finds a suitable location -- ideally behind your walls.
3) The unfinished signs are to prevent people from landing inside of the vault. It adds more obstacles to prevent landing inside of the vault with a bug jump.