Double Shock really evolved from it's original plan into something completely different.
At the beginning of Double Shock, Dexter was cooking with Mrs. Peck and telling her to take these pastries over to her sister's. So I think they were planning for Mrs. Peck to be gone during the murder.
Then she changes her mind because "she didn't want to leave Clifford alone".
So that was the 1st adjustment: With Mrs. Peck now home, they had to quickly replace the fuse.
Next was the autopsy. I think they were hoping to get the entire death brushed under the rug as a "heart attack", but the autopsy showed otherwise. So now they tried to implicate each other to confuse Columbo, and he wouldn't be able to figure out which one of them did it.
Finally the will, and supplemental killing of Lisa Chambers. They didn't know beforehand about the will, giving everything to Lisa. So now they work together to frame the lawyer for both killings instead.
So what started out as a simple Home-Alone heart attack, really evolved into quite a different story at the end!
Good observations! Was Mrs. Peck planning on taking the pastries over to her sister that night? She did put her coat on, but got caught up in the soap opera on TV. Which brings up a question: Isn't it kind of late for soap operas to be on TV? I mean, this was before the days of tape-delay, late night soaps (Dallas didn't air until the late 70's; it was the first night time soap since Peyton Place ended in 1969).
I don't think it was literally a soap opera. One of those dramatic doctor shows like Medical Center.
Could be, but the production value was obviously of the cheap, daytime soap opera style. Gotta love the cheesy soap opery dialogue, too: "If you're not cold and hard and tough enough to justify the faith and confidence of your patients, then you can hand me your stethoscope, Rocky." I always get a chuckle out of that scene, although I don't think that's what they were going for.
What's also interesting about that TV scene is that four actors are credited on IMDB: Young TV Lawyer, Young TV Doctor, Senior TV doctor, and Older TV Lawyer.