Question About Copyright Ownership Of An Unpublished Biography Manuscript From 1989

Location: Michigan, USA

I'm looking for guidance on potential copyright issues before self-publishing a book.

My uncle passed away in 2010. Recently, my aunt gave me an unpublished manuscript that was written in 1989 by one of my uncle's close friends, who died in 1992. The manuscript is a biography of my uncle's remarkable life.

As far as I know:

• The manuscript was never published.
• It appears to have been an early draft typed on a typewriter and sent to my uncle for review and edits. It was missing several pages and was heavily marked over in pen.
• The original author died in 1992 in Iowa.
• The author had no children. I'm unsure whether he had a spouse. His siblings are deceased.
• The manuscript remained in my uncle's possession until his death and was later inherited by my aunt.
• My aunt voluntarily gave me the manuscript and encouraged me to finish and publish it.

Over the last year, I substantially revised the manuscript. I:

• Reorganized chapters and chronology.
• Rewrote much of the language and syntax.
• Added new material and context.
• Created what I consider a significantly revised version rather than simply retyping the original manuscript.

However, the original manuscript remains the primary source for much of the biography.

My questions are:

1. Who likely owns the copyright to the original unpublished manuscript?
2. If the author's heirs can be identified, would they likely have rights in the manuscript even though it was never published and remained in my uncle's possession for decades?
3. If I wanted to proceed with self-publication, what level of due diligence would typically be expected in locating potential copyright holders?
4. If heirs are found and do own the copyright, would they potentially be entitled to a share of proceeds, and how is that usually handled?
5. Does the fact that this is a biography based on real-life events (rather than fiction) materially affect the analysis?

I'm not looking for specific legal representation, just trying to understand the copyright and inheritance issues before deciding whether to invest further in publication.