Question: McPherson broke the grandfather clock and nothing was in there. But then, the next time he opened it with Laura there, nothing was broken, and there was a gun in there. How did it get fixed? What did I miss?
Question: Am I missing something? How is it possible for McPherson to fall in love with Laura?
Answer: While investigating her supposed murder, McPherson interviews many people who knew Laura and gave their personal accounts about her. The more McPherson learns about the beautiful woman, the more intrigued he becomes, eventually almost to the point of obsession. He was more infatuated and physically attracted than actually being in love with her.
Answer: It's a different clock. The first one is in Lydecker's flat, and there's an identical one in Laura's. McPherson finds the secret compartment in Lydecker's clock, and since Laura's is the same, he checks that one, too. The shotgun is in Laura's clock.