Pardon me for being a skeptic, but I'm not convinced that Apple's diagnostics are showing nothing wrong. Too many users are having the same problem for Apple to be seeing absolutely no issues in their diagnostics. It's just easier for them to deny it, perhaps while they try to figure out what's going on. After they know how to fix it maybe they will admit that there was an issue.
And the reason that the problem is not specific to a particular Apple watch model is that the issue is a software issue, not a hardware (watch) issue. This is clear given that before the "upgrade" to 10.0.1 no one encountered the issue. The variation in the severity of the battery drain can be explained by what apps people are running on their watches, how much they use their watches, whether they've got apps running in the background or only when in use, and things like that.
I can't explain why some people see improvement when they try suggested fixes; this could be related to the variables that I just described. They seem to be temporary improvements, and not permanent fixes.
Don't buy a new or refurbished watch. You'll just have a different watch with the same OS if they're shipping watches with the current OS, and you'll have the same issue because it's not the hardware, it's the OS. Unless, of course, you get a watch with a version of the OS prior to 10.0.1, and then ensure that you do not have it set to do automatic OS updates!
The bottom line is that there is a problem with the software, and Apple undoubtedly knows about it, and they are responsible for fixing it. But will they, and how soon?