How can I tell when the work was first published? Let's be real -- you probably won't be able to determine first publication dates. However you can probably make a few assumptions. The only way a work from the 19th Century would still be protected would be if it (1) was first published after 1977 and before 2003, in which case the work is protected until December 31, 2047 (notice is required for works first published before March 1989), or (2) was first published after 2002 and the photographer died after 1943. There are some other possibilities for continued protection and you can figure them out using the public domain chart. But generally, these scenarios are fairly unlikely. As you're also aware, the Library of Congress makes an effort to explain copyright status -- for example, as with this collection of daguerreotypes and with this general explanation.
What should you do? Your references to case law are impressive but unnecessary. Your use of the photographs may be transformative (we're not so sure) but you won't want to be dragged into court to prove your point. And more importantly, whoever dragged you into court would have to (1) qualify as owner in the face of the public domain rules and (2) show a clear chain of title dating back to the 19th Century. That is, to sue you, a copyright owner would have to prove that the rights descended through inheritances, or were passed along to the owner through a series of copyright assignments. If anyone has heard of copyright holders asserting such rights in 19th Century photographs, let us know ... but until we hear otherwise, the risk of creating portraits seems fairly low.