I don't think you could actually call the Gerard/Rossi situation blackmail as it's actually more like extortion. Gerard was extorting the three restaurateurs with bad reviews if they didn't pay him. The Santini/Jerome situation would actually be considered blackmail since Jerome knew Santini's secret and was threatening to turn him in if he didn't pay him.

As far as the Danzinger situation goes, he was married to a wealthy woman, but I don't think it's ever stated if she fronted the money to get Danzinger's business started. She does seem like the shrewd type that would definitely make sure Hayden paid, and paid dearly, if he betrayed her in any way, so he had plenty to lose. I have to wonder, though, just how many different men had Rosanna Wells blackmailed in this fashion.

That’s true..
When I think of Rosanna Wells, I think of a person who is walking a tightrope and in dangerous territory, much like a person who sells drugs…. However, in The Most Dangerous Match when Berozski was killed, I felt bad for the guy, he was certainly a nice guy….. and there are plenty of examples of nice guys who were murdered. Howard Nicholson, Jim Ferris, Alan Mallory and Dr. Henry Willis for example….

And did Edmund really deserve his just rewards in Try to Catch Me? After all, there was never any conclusive proof he DID kill his wife. It really COULD have been a terrible freakish accident. However Abigail had made up her mind, brooding and dwelling on her niece's death, and in the very first scene it was obvious he was going to die, deserved or not.

Perhaps you’re correct, but remember when Columbo went to Edmund’s apartment and couldn’t find a single photo of his dead wife? It’s conjecture, kind of like when Hayden Danziger told Columbo, “Lt, I thinking that falls in the area of speculation”