#4: I think "erasing" the board would be more commonly used than "wiping". And it bothers me too that Columbo didn't finish erasing.
#6 and #7: Good points.
#8: Maybe they would just pour the concrete without looking that far down in the hole? Maybe it would be dark enough at the bottom to conceal the body?

and don't forget Elliot's Secretary, the first visit by Columbo, whenever Columbo makes a statement about Elliot or his office, ..."yes, it is" ... how many times in a row does she say, yes, it is?

She says it at least 3 times, maybe even 4, in the italian version it's slightly different: "Sì, tenente" (yes, lieutenant).

Hey Esploratore, do you have dubbing in Italy? or you hear the original voice in the background?

Hey, yes, there's italian dubs for all episodes, which is how I watch them, I don't hear the original voice in the background, columbo himself has been dubbed by 2 actors: one for all episodes until season 9, then another one following the former actor's death, they had such a similar voice that I didn't even know they were 2 actors till now.

The only exception was ransom for a dead man, which was with a different actor, much different voice that episode.

And you, do you watch it in the original language?