Well, Whismicott is often not running Encore, especially when there are way better answers to Zangoose that you could run. Also, Liepard isn't run that much, usually it's used as a Rain setter if it's used at all. Most Ghosts check it well, although Mismagius is required to run a z-move different from Never-Ending Nightmare. Zangoose being frail isn't really an issue if you give it Parting Shot or Memento support, as well as if you put potential checks in Quick Attack range. Oftentimes, you won't even die from priority, even Mach Punch from Choice Scarf Hitmonlee. So, I can't really agree with the points you make in your argument. Zangoose will fail if you don't give it the right support, but if you do, the flaws you list above, and even some of the ones I list, won't really be a problem.
Whimsicott runs Encore pretty darn often and if it doesn't I must be really lucky finding them.
I didn't want to portray that Liepard was a relevant counter to BD Zangoose, I wanted to say that Liepard can be used to lock Zangoose into BD if the situation calls for it. I know Liepard can only find a use on weather teams, but I'm still noting it as a threat to BD Zangoose.
Yah no in a situation with Scarf Jolly Hitmonlee vs Zangoose with no hazards and Zangoose already Belly Drum'd (aka its at 75%)
252 Atk Hitmonlee Mach Punch vs. 0 HP / 0 Def Zangoose: 188-224 (65.5 - 78%)
Yes this is a small KO chance. But we all know Black Belt and Life Orb Hitmonlee is also an often thing to run, and if that Scarf Hitmonlee had Adamant instead of Jolly for whatever reason that Zangoose is most likely gone. I'm not saying Toxic Orb won't get shut down by priority, but it isn't shut down as bad.
252+ Atk Black Belt Hitmonlee Mach Punch vs. 0 HP / 0 Def Zangoose: 248-294 (86.4 - 102.4%) -- 18.8% chance to OHKO
Even with poison damage Zangoose can survive this situation.
Don't take my previous comments as a way to devalue Belly Drum. Belly Drum is a lot more effective set. I just find BD to be too much of a high risk high reward option