Maybe Yanki J is a former part of the mafia who Critic meets up with through JewWario and he knows the killer also used to be part of the mafia? And this leads to Critic meeting up with MobBoss!Diamanda to get info?
I’m just rambling here. *shrug*
Canon. (Suggestions for how JewWario knows the Critic are open, though.) Yanki J gets killed by the murderer before he can give any other information about the killer. This forces the Critic to go to Diamanda for info.