Yes, I would say Mary Maloney's actions and reactions after the murder are believable.

We learn that, after she whacks her husband on the back of the head with the frozen leg of lamb (an act done in an almost trancelike state), she suddenly can think very clearly and begins to think "very fast." As the wife of a detective, she knows the penalty for murder, and we surmise she also knows something about crime detection. At first, she believes she is willing to pay the price for killing her husband, but then she remembers her baby and wants to protect it.

It seems very plausible to me that a pregnant woman would be so devoted to her unborn child that she would want to protect it by getting away with the crime. She doesn't want to risk being separated from her child.

It also seems plausible that she would feed the detectives the leg of lamb. She is a domestic creature through and through and is doing what comes naturally to her. She has also been thinking very quickly and is intelligent enough to understand that this is a good way to get rid of the murder weapon.