What could be the possible questions that the detective could have asked to Sam, the grocer?
I think that the first answer has most of the possible questions. I think that the questions about Mary's state of mind are the most important.
Because we have read the story, we know that Mary has rehearsed what she is going to say to Sam. We know that she is going to try hard to make him notice her and remember that she was there at that particular instant. You would think that this would make her act in an unnatural way.
So I think the detective really should ask Sam about how Mary acted. He should ask if she seemed like herself. He might even push and ask specific questions -- did she seem excited, did she seem like she was trying to make you remember her, did she seem like she was playing a rehearsed role?
I think that the question about what she was carrying is a little less useful -- after all, I doubt she would go to the store carrying the murder weapon.
We know already that her reaction, according to Sam the grocer, was normal. Therefore, that was the first question or primary question that we must infer that the detective asked Sam. In addition to whether her reactions and actions were normal, the police should emphasize on the time that she made it to the store and compare it to the time of death of Patrick Maloney. That should give them the key to the puzzle.
Other questions could include: Was she in a hurry? How long did she stay (for the sake of her own distracted mind), What exactly were her words? Does she often explain to you what she is going to do with the things she buys here? Are you on a daily-speaking basis?