I really enjoyed this! Your characterizations are excellent -- Dung and Hestia and Dedalus all just come alive, with superb voices and spot-on reactions. Mundungus is still clearly his canon self, but you've shown, believably, how much more there might be to him. A good con-artist has to be shrewd, and he is: noticing the connection between speaking Voldemort's name and the appearance of DEs, figuring out the electrical issues, understanding how easy it is for people (like wizards) to underestimate the culture of people unlike them (and so not to realize, as the DEs don't, the potential for Muggle-style physical violence.)
These are my favorite sorts of Beholder stories: ones thick with unexpected, plausible characterizations and wizard-world details.