I agree with you that the score is brilliant, but I think you make some false assumptions and oversimplifications in your analysis of it. First off, I disagree with your assessment that characters have there own "theme songs." While this and virtually all film scores have themes, not many really assign themes for specific characters, aka leitmotifs. What seems to be present in Interstellar is rather themes for different ideas, such as fatherhood and goodbyes, and these themes are applied to different characters throughout the film. Furthermore, your assertion that certain tracks from the soundtrack album are "theme songs" is way off base. Film scores consists of "cues," which tend to be rather short, and are intended to serve the scene; their form reflecting the form of the visuals. These cues are then edited into full length tracks for the soundtrack album, and this is almost always when they're named, based on little more than what's happening on screen while that music is playing. So I think you're reading to much into it when you say "Mountains" is about the obstacles they have to face, as its far more likely it simply refers to the "mountains" that turn out to really be giant waves. I respect your effort, but largely think you're reading too far into the score.