The chests location is hinted at in Journal Page 8. It’s location is very memorable at night but you need to dig it up during the day. That chest will then lead to answering all of your other questions. If you haven’t already, make sure to bring a soulfruit to the tree that wants one. You will receive an important recipe for getting that key.
The Manor chest requires the West Wing key to open the door and the East Wing key to open the chest. You have most likely already obtained the item located with that chest and used it for it’s intended purpose. If I recall correctly, it also contained a (Filler) story item.