Mr G is one of those studio gurus....and you have no idea what he can do with the toys he has. Just last week he manufactured a word for a jazz song he was mixing. The singer left the word out all together, and he has it being sung on the final product. Even he, as an old pro, was impressed.
There is software that will allow you to tune, edit, (time or words) or even replace one voice with another. I know one producer who sings a part, then replaces his voice with that of the “singer” (who isn’t too good). That way the song is in tune, in rhythm, and with “heart”. Its the only way they got the album done. (and no, I will not tell you who it is)
With the amount of material out there with Mel’s voice on it, this tape could be made up out of whole cloth if someone wanted to.
Mr G could put those tapes together here, without ever having met the man or spoken to him. Just getting hold of his movies would be all he needs, to get his voice. Analyzing them won’t tell you much, unless the guy was really sloppy about it. If you don’t have the original files (nobody uses tapes any more... all done in computers)it is hard to tell where the editing has been done.
Visualize what computer graphics has done for animation and movies, and photoshop for pictures. You can do all that with audio as well.