Source: Archiwum Państwowe w Gdańsku
I have gathered quite a big collection of the members of the Tolkyn (Tolkin) and Tolkien family. What I can say is that the form Tolkien is much earlier that the form Tollkühn, and it is contrary to the claims of J.R.R. Tolkien himself! The form Tolkyn occurs in the 15th century, Tolkien occurs since 16th c. and Tollkühn occurs very late, in the 19th c.
In the parish books of the St Mary and St John churches in Gdańsk I have found the proof of the "Germanization" of the family name which from the earliest times occurs as Tolkien (a family name of the Baltic-Slavic origin). In 1814 a Dorothea Elisabeth Pelican, née Tolkien*
becomes Dorothea Elisabeth Pelican, née Tollkuehn in 1816
Tollkuehn is of course another graphic form of Tollkühn, and tollkühn is an adjective 'fool-hardy'. Such "folk etymology" or rationalization of the non-German family names occured often in the Prussian period of the German history.
* who was probably a sister of Daniel Gottlieb and John Benjamin, daughter of Christian Tolkien and Eufrisina)