Question: Maddy pulls up to the house in her dead husband's Cadillac right around the time Ned was supposed to be at the boat house. Why would she be driving his car when at that point in time, she is supposed to be Mary Ann Simpson, her real identity? Ned was supposed to be dead at that point. The boat house could have blown up as she arrived and the fire dept. called by a neighbor would be on its way?
Answer: Maddie (who was the real Mary Ann Simpson) was deliberately being careless, dropping clues, and creating inconsistencies, like leaving Edmund's eyeglasses behind at the house when they moved his body. It was the same with Edmund's car. She was framing Ned as being the sole murderer who killed both (the real) Maddie and Edmund. She had specifically targeted Ned as her accomplice, knowing he was an incompetent lawyer who was careless about details.
Question: If Ned couldn't open the boathouse door and got blown up at the end - how did Kathleen Turner do it and survive the explosion?
Answer: It wasn't revealed, but since she was the one who orchestrated the plot, there was likely an inside escape hatch for her to get out of the boat house before it exploded.
Answer: Timer.
Answer: It was all part of the plan to frame Ned, she purposely left clues behind as Ned would become suspicious of her. In the end, the dead body of the real Mary Ann Simpson was already in the houseboat. Thus everyone would think she's was dead when in fact she in sunning on a tropical beach.