Patzer

The key on the corpse's necklace and the one in the flashback are clearly two different keys (one is round and the other is rectangular in shape).

48 minutes in, when Isaac is looking at the "corpse" in the basement through the hole in the wall, he shines his flashlight on her face, and when he shines the light on her eye, her pupil dilates. This would not happen in a corpse. In fact, doctors check pupil dilation to see if a patient is brain dead. And the pupil will already fully relax (dilate) after death. This and the "corpse's" slow but very visible breathing later on are subtle clues foreshadowing the final twist.

The leather harness Isaac is wearing has two large buckles right behind the shoulders. It looks like it would be easy for him to simply reach behind each shoulder, undo the buckles, and wiggle out of the harness, but instead he concentrates on trying to break the chain.