Is the house used on Fade in To Murder as Wade Fowler's house the same house that was used for MacMillan and Wife as their house?

No. Shatner's house was known as "The Enchanted Hill", which was built for Fred Thomson and Frances Marion (see this amazing link for more information on the once beautiful lot: https://paradiseleased.wordpress.com/2011/09/05/lost-hollywood-the-enchanted-hill-of-fred-thomson-and-frances-marion/). "Identity Crisis" and "Double Shock" were also filmed at this location. They did film part of "The Rock Hudson Story" at "The Enchanted Hill", so perhaps that's what you were thinking. I think one of the things which drew me to Columbo right away were the terrific filming locations - this being my favorite one.

I'm not so good at making up trivia questions, but I've always noticed that Fade In To Murder and the movie THE CHEAP DETECTIVE have one very small thing in common (apart from Peter Falk). Can anyone guess what it is?