I was able to easily figure out Dr. Croaker's true identity by his voice. When I first heard him speak early in the episode, I recognized Charlie Adler doing his voice. So then I listened for another likely suspect that Charlie Adler also voiced, and then
when Wendell McWendell spoke when he came on I was like "That's the villain!" Charlie Adler's voices are so distinctive, it's impossible to miss them. (It probably also comes from years of watching "Tiny Toon Adventures," "Cow and Chicken" and "Rocko's Modern Life", where he was a prominent main voice actor in those three shows.) Though Charlie Adler also voiced that boy Mr. Cashmore ripped off (basically using his Buster Bunny voice from "Tiny Toon Adventures"), I figured he couldn't be the crook as he was such a minor character in the story and a bit too young to have pulled off the heist. Wendell McWendell seemed more likely to do it than that boy.