$200MM is a bit high, even for Ansel. For dem kind of rupees we're talking Rembrandt.
Anyhow if the dude owns the plates, then he owns the copyright. These plates were displaced eons ago, may have even been sold as seconds or thrown away. The dude bought them fair and straight. Thus he owns the copyright as well.
If it took 10 yrs to "prove" they were Ansel's work sounds fishy given that Adams has signature work. The museums aren't always wrong.