My post on the Darwin photo was picked up by Nick at the Panda's Thumb. Yesterday, I contacted Kevin Repp, Curator of Modern European Books and Manuscripts at the Beinecke and I received this reply today:
Many thanks for your query. As it turns out, the photo in question is indeed of Mr. Princep. How it came to be cataloged as one of Darwin is a mystery. The inscription business makes it all the more curious. In any case, I have alerted our digital library cataloging staff, and the record will be corrected promptly. Thanks again for pointing this out to us.
Mystery solved ... well sort of. We still don't know about the inscription.
Princep was a British civil servant in India and was married to the photographer's sister. See here (which reproduces an 1865 photo that is clearly from the same sitting).