5) I have a feeling it was etude in black, I don't remember if he actually revealed his salary in that episode, but I remember well that he was surprised about Alex Benedict having a huge house, nice car etc. and made some calculations and asked Alex how much he earned, and I have a feeling he also revealed how much he earned himself, explaining he could never afford stuff like this with what he makes.

9) Has already been partially answered, but Barsini is the killer who painted columbo

11) The fact Cassidy starred in 3 episodes has already been answered, but I wanted to say they should be murder by the book (franklin), the great santini and publish or perish (greenleaf)

12) Was never a columbo episode, I was surprised and thought I had forgotten an English title (what with me being italian and watching columbo in italian), but after that option appeared I was pretty sure!

Yes, #5 is Etude in Black. He says to Alex Benedict, "Now, I make $11,000 a year... It would take me 90 years just to buy a place like this."

However, in Death Lends a Hand, Brimmer says, "I can almost guarantee I can triple your salary in the first year." And later, a Brimmer associate who was chewed out for giving out too much information, tells Columbo the top guy can haul in about $30,000 a year. So, one can deduce that Columbo makes in the vicinity of $10,000 a year. Maybe he got a raise between the Brimmer case and the Benedict case.