It doesn't add up. So if it's true that wade was testing her: if she bites it would be right to shoot her, but she didn't - so why load the gun. Also why was it important that the gun isn't loaded and why sleep with it. Also, why was the cartridge on the floor? Is that a normal place to hold cartridges? More probable it fell down for some plot reason.
shareWade dropped the shell when he came into the house and found Maggie seemingly passed. I would guess in such a shocking moment, he didn't notice he dropped the shell until the next morning as he walked out to find her.
Based on his reaction upon waking up, I don't think he was testing her. I think he was asleep, though didn't intend to be.
Wade was awake the whole time. When Maggie puts her face close to his as if to bite him, the camera cuts to Wade's hand on the gun and his finger moves just slightly. Then, after she leaves, he fully opens his eyes.
shareIt's a movie
A good one at that
I think the point was how deep inside he knew he couldn't pull the trigger when the time came.
She smelled him. You can hear her sniffing him. That's when she knew he wouldn't be able to keep his promise. With what little life she had left she kissed him goodbye and went to the roof. When she turned she knew she'd step and spare her father what they both know he couldn't do.