You're getting some misinformation about copyright here. In the U.S., owning the manuscript does not mean you own the rights to it. It doesn't matter whether the author ever registered her copyright with the LOC or published the book. The rights to do anything with it remain the property or her, or her estate or heirs, for her lifetime plus 70 years. I assume that if her worldly goods were being sold when you purchased the manuscript 15 years ago, you've only got 60 years more to wait.

Your heart's in the right place. Seek her family and give it to them.