How Can a 100 Year-old Autobiography Still Be Under Copyright?

Dear Rich: I am writing a play about a real woman who lived from 1839 to 1930. She wrote an autobiography which was not published in her lifetime. In 1994, it was finally published after some editing by a family member. I am using a lot of the original wording from her autobiography in my play because her voice is really interesting. I’m sure that I’m going far beyond what would be considered fair use so I would like to get permission from the copyright holder. Unfortunately, the publisher has gone out of business and the editor/relative has a very common name, so I can’t find him. He could well have passed on by now. I’ve tried to track the relatives, but the trail is cold. Would her heirs still hold copyright in her words at this point? Would the relative who edited her autobiography for publication have some copyright? Is there a best way to prove I’ve shown due diligence in attempting to contact the copyright holders?
Copyright protection typically lasts for the life of the author plus 70 years. So, you would think that copyright for the autobiography would have expired in the year 2000. But the autobiography is still protected because of a quirk in the law that says if an unpublished work created before 1978 was published before January 1, 2003, its copyright would last at least until January 1, 2048, regardless of when the author died. That may have been the reason the relative edited and had the book published in 1994.
Getting (or not getting) permission. Considering the work is under copyright, you'd need permission from the copyright holder (unless you can demonstrate it's fair use). We guess that the relative who edited the work holds the copyright (or his heirs). He apparently instigated the edits and the distribution, so, hard as it might be, you would be tasked with tracking that person down. As you're probably aware "people finder" web searches are available for a fee. (BTW, we don't believe that basic book editing amounts to a separate copyright.)
If for some reason, there are no heirs and after diligently searching, you can't find anyone to ask for the autobiography rights (referred to as an "orphan work"), then you have to decide whether to proceed without permission. If you do proceed, keep a record of the steps you took to find the owner. Those records could minimize (but not eliminate) damages in the event the copyright owner steps forward. Finally, if you have success and a company wants to produce your play, you may be required under the contract to clear all rights in which case you may have to hire a researcher or clearance service.