To presume that Tolkien would have had an understanding of the then state-of-the-art theories around continental drift is a bit hopeful. In fact, plate tectonics did come to be accepted until the 50s/60s, so Tolkien could barely have known of the theory when writing LOTR and especially not when drawing the initial map.
[1] https://en.wikipedia.org/wiki/Tolkien%27s_maps#The_Lord_of_t...
The mapping in The Hobbit was finalised in 1936 and published in 1937, and was started in the late 1920s. Detailed maps of Middle Earth for TLOTR were produced in the 1940s, e.g. a contour map of Minas Morgul from 1944, although I can't find a date for the first rough maps.
To my mind there is a bit of a tension between Tolkien's "I started with a map..." comment and the fact that the story took a while to settle [0] down on the core theme of the One Ring and the need for a quest through Middle Earth (hence the map) that would destroy it.
[0] https://en.wikipedia.org/wiki/The_Lord_of_the_Rings#Writing
[1] https://en.wikipedia.org/wiki/The_Silmarillion#Development
Yes. To be honest I read several of CT's books on the history of Tolkien's writing but intentionally stopped when he reached TLOTR because I didn't want to see one of my favourite books deconstructed, e.g. the 'too many hobbits' versions, before the hobbit Strider became the ranger Aragorn, for example. I suspect reading those volumes may make the interrelationships clearer but I probably won't do that.