Q:
How do I get into Garrett's house?
A:
Try inspecting the houses around the one Garrett lives in.
There is one that has a window facing the front entrance of Garretts home.
From there, you can see a window with closed wooden shutters.
Hop over the street and open the shutters - and in you are!
Q:
How do I get into the graveyard?
A:
Don't bother. You are not meant to get there in this mission.
It is said that there is a wall you can mantle to get there; but do so at you own discretion.
Q:
In this level, I have pulled a lever in a brightly lit room with a
little table and chair (if i recall correctly) and can hear a gate opening.
Where is this gate and how do I get there?
A:
"Right below this place." (The gate that opens is right below the room with the lever.)
Q:
What happens if i don't pull the lever?
A:
If you don't pull the lever, the gate doesn't open.
Q:
Do you know where the wall to mantle over to the graveyard is?
A:
Yes. You'll need many boxes and stuff; it is a bit complicated, a one-way trip, and there is nothing of value or interest to be found in the graveyard.