I firmly believe that Lee wrote the story “Watchman” and gave the manuscript to Truman Capote to review/edit.
Capote revised, edited and cleaned up that story and came up with “To Kill a Mockingbird.”
Capote was her lifelong friend, and he was already famous, so he gave this to her as a gift and a hand up.
Right there with you on Capote being the voice behind “Mockingbird”. I think it explains why Harper Lee essentially disappeared for decades.
I surprisingly liked the 2005 movie Capote in which about 1/3 of the scenes have the Harper Lee actress with him. Once they even have a quick kiss on the mouth despite there being no romance, just friendship. The movie shows her completion of her novel and then quick fame surpassing his.