Hearthfire is pretty buggy and never had a (official) patch, things appearing in the walls could be from the spawn points messing up just outside the house causing things to spawn in the walls torch bugs bandits even giants. Items disappearing depends on the item unique weapons can pop out of racks and slip though the floor rarely npc's in the house can also pick things up, the deaths and random child i have no idea. Mannequins are just weird even without Hearthfire they seem to be a npc programed not to move but the game sometimes forgets.
On the PC check your mods don't conflict and the load order is correct try the unofficial patches if you haven't maybe even clean the dlc. On a console all you can really do is re-install the dlc personally i would start a new game first and see what the house is like in that before doing that. Sometimes it seems like it a random chance if Lakeview and the other houses are extremely or just slightly glitch, on a new game it maybe better.