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Logically, yes, because every time we post in musing, it will automatically enter the "my comment" tab in steemit, and the person you tag can see the data tag using several applications such as SteemPlus, in the mention section.

A lot of things don't work quite like they do on Steemit if you post on Musing, like HTML and Markdown, and also URLs don't automatically link. So logically it's in question.

Empirically, one of the two I put in the post worked, and the other didn't, which is really weird.

Hm. Well, I got a Steemworld notification to this account, but none to @tcpolymath. They both look well-formed on Steemit. We'll see if the comment works.

It did on Steemworld but not on Gina, weirdly. I suspect that as long as the mention isn't the first thing on the line it probably will work.

Yes they do. As i have explained in some of my previous answers, question and answers here in musing are contained as posts or replies to posts in steem blockchain. Now if you mention/tag someone in a post or a comment they get notification . And as musing Q&As are simple posts and comments they should get notified.