Please note: all credit goes to FrozenShiver!
All of the achievements are storyline. The knight isn’t scary since he has a long windup before he attacks. But if he still bothers you, go to the kitchen or main entrance door looking to the outside. He can’t normally pass through them, but if you bait his attack, the attack animation will get him stuck outside and you can now roam the house freely.
Enter the house, at the bookshelf on the left, take the note, this wakes up the knight. (he doesn’t move while you read notes).
Head to the kitchen (left of the piano), turn on the light next to the clock, interact with the clock and hold the right mouse button until the key appears, pick up the key.
Next, go upstairs, the first door on the right, read the note at the table, now in the hallway a trap door to the attic drops down a ladder, up there, open the window on the right and get the key.
Downstairs, you can now open the room to the right of the piano, pick up the key on the shelf behind the door (there is a note here, but it isn’t required).
Then go to the basement (the door on the right of the stairs when you enter through the main entrance), pick up the seeds on the table.
Now go outside, behind the house, at the grave, Press F to pay respects interact with it, wait for the flowers to grow and pick up the key, you can now escape at the fence with the car.