Very interesting question, specifically about the head clamp.

As for the daguerreotype, I think that a lot of folks not so versed in composition would think that placing the faces right smack dab in the vertical-middle made the most sense. There are some Brady portraits like this that I've seen, and it looks awfully balanced to my eye.

As for why it wasn't rotated on the horizontal... who knows, maybe his camera & tripod literally wouldn't allow for it.