If this is not a bug and it’s intended to work this way, it will change the way I, and other players, handle future Seasons. I find other Class Set pieces often, when playing my main Character. Usually, I just salvage them. Now, perhaps I’ll start stashing them instead. 
It also means that many online Guides will need some serious updating. 
Yes. But, I think it’s only when the username is preceded by the ampersat (or “at”) symbol @. (Maybe that’s what you meant by “mentions” your name). I am also not sure if using punctuation will cancel that feature.
Perhaps it can be tested sometime. You are welcome to use Perusoe’s Testing Grounds to do so. I may even try it myself, if I get up enough energy.
I would suggest using the “Reply” button at the very bottom of a thread to avoid replying to any particular post. Hopefully, that will eliminate the possibility of it notifying the user because of the Reply. (But, you probably already thought of that).
In any event, thanks for sharing. 