score:6
The obvious place to look would be in the letters from Constanze to her husband. Unfortunately, as far as I am aware, none of these survive.
The letters of Mozart and his family were collected (and translated) by Emily Anderson. These were published in 3 volumes in the 1930's.
I was able to find the first and third volumes on Archive.org. As far as I could see there is no use of "Wolfi" or "Wolfie" as a pet name for Mozart in any of the letters.
The second volume is on Google Books. A search inside the book didn't identify any occurrence of "Wolfi" or "Wolfie".
It looks like it may well be the case that they used the film as a source in this instance.
Upvote:1
Mozart’s childhood name was “Wolferl”, a south German and Austrian diminutive of “Wolfgang”. “Wolfie” is merely an attempt to translate this into English.
http://wwwg.uni-klu.ac.at/kultdoku/kataloge/31/html/2461.htm