You can absolutely learn enough Mandarin to be useful in 2 years of weekly classes.
Source: That was my level when I went to China. I felt not at all ready, but then I discovered that my limited knowledge could get me quite far with a little effort.
Useful isn't an objective metric for language competency.
Cramming for a few weeks before visiting a country is useful.
It takes the average fully trained foreign service officer who already speaks multiple languages about 2200 hours to become proficient. And this is 2200 hours at arguably the best language school in the world doing nothing but studying Chinese full time.
2 years of of Weekly classes is about 100 hours of class time.
You were the one who was talking about usefulness. While it may not be an objective metric, it's still what everyone evaluates their language ability on: whether they can use it to talk to people.
The bar for diplomats is much higher, since they need to be fluent enough to avoid a diplomatic incident. Would you trust someone with 2 years of high-school French to speak that well?
That said, I agree that Chinese is more difficult than most other languages; mostly due to the writing system, which even Chinese schools spread over multiple years for good reason. But that is no argument against offering the language anyway.
Source: That was my level when I went to China. I felt not at all ready, but then I discovered that my limited knowledge could get me quite far with a little effort.