Mozart, Gershwin, and the Two Michaels

In further investigation of the status of "Lily Pons" of Gershwin, listed as melody no. 79, as I had mentioned I was undergoing in a recent blog, I have decided to terminate, at least for the moment, any additional looking into the status of this music until I am in contact with the Library of Congress.
I found that Gershwin never completed the piece, which is recorded by Michael Tillson Thomas. As the recording divulges a piece which is a complete statement, I can only assume that Thomas himself must written the completion into this music, which he then recorded. Final determination may be available in the Library of Congress.
If Thomas did indeed complete the Gershwin fragment, it brings back another Michael, whose last name was Sussmayer, who completed Mozart's Requiem and then presented it to Mozart's widow.
SO! - In both the Gershwin and Mozart cases, which were the last notes the two composers wrote, and where did both Michaels take over for the respective completions?