Mim already figured as a
dwarf in the first version of the 1915 Tale of Turambar and in the
Nauglafring, when all (!) dwarves
were still perceived as being evil by JRRT. The idea of Seven Houses with Seven Fathers was only to arise decades later.
JRRT for the first time wrote down the idea of the dwarves being created by Aule - therefore not being altogether evil, though not having
spirits like elves and men :) - in the second version of the later Annals of Beleriand (1931 or later).
I would like to point out that throughout the pre-war development of the story of the fall of Nargothrond and Doriath, Mim has always
been named Mim the Fatherless (!) by Tolkien. He played a major role in these stories, his curse causing whole chains of events.
As I said earlier, Mim was an idea from JRRT's youth, when he was still under the influence of other mythologies. Mim survived the
revolutionary changes JRRT made to the stories relating the sack of Nargothrond and Doriath and the changes to the origin and character
of the dwarves.
As such he was changed from Mim the Fatherless Dwarf to Mim the Petty-Dwarf, last of his kind. But I do not know of any effort of
JRRT to explain explicitly the extraordinary place of Mim in the context of the eventual world of the Silmarillion.