It's hard for any of us to imagine ourselves in the same situation. Try to imagine yourself confronted with a neighbor's relative telling you that your neighbor hasn't been seen or heard from in two days. That relative asks if you will accompany them in searching the neighbor's house. You agree and during the search you find your neighbor dead - naked in her bathtub, with blood around the drain. Imagine how you'd react. Do you think you would ever forget any details of that night? It's the stuff that nightmares are made of, and not something you'd easily forget, even though you wish you could.
As humans, we tend to remember with vivid clarity the major, out-of-the-ordinary, events in our lives. Most of us won't ever find a dead body - murder victim. The closest we might come to the same shock and vivid recall is the events of September 11, 2001. Each of us here can probably remember in detail, where we were, who we were with, what was said, and how we reacted when we heard the news that terrorists had flown airplanes into the World Trade Center. It's that kind of shock, but on a more personal level, that one would encounter in finding a dead body.
With Steve Carcerano adding or changing details, and reciting the story as if it were a well-rehearsed monologue for a play, his story has all the appearances of a cover story. The big question is why would Steve cover for DP? It has to be something big. DP fixing Steve's gambling debts has been suggested, but I think it's something bigger than that. Steve is somehow indebted to DP for something major.
Do we know for sure that there was actually a "Mary" present when Katheleen was found?