Regarding your last point: if I remember the foreword to the 2nd edition Tolkien describes his writing process as essentially being spatially. He moves his characters from place to place, and seems to only have put in a core theme later on. This also agrees with the whole "Tolkien wrote a book so he could talk about cultures/languages/something" theme. This would also suggest that he had in his mind something of a map or layout of Middle-Earth very early on. And than there is the complicated interaction with the Silmarillion, which was developed long before the Hobbit even (1914), but also during writing of the Hobbit and TLOTR. [1] So disentangling where exactly he came up with what seems almost impossible (and something that many Tolkien scholars, including Christopher Tolkien, have spent quite some time studying!)

[1] https://en.wikipedia.org/wiki/The_Silmarillion#Development