Jamie,
I have to agree with you about the belt over the waistcoat thing-- In this instance Lord Mansfield is wearing his uniform as an officer of the Atholl Highlanders, and that's how they do it. If one just had to show off his belt plate then this would be acceptable civilian attire, but preferable attire would have the dirk worn suspended from a small belt fastened beneath the waistcoat. A much tidier look in my opinion.

I suspect the hilt of Mansfield's dirk is tucked under his jacket to prevent it getting in the way while dancing. Judging by its looks that particular dirk has seen some hard knocks!