I have been recently enjoying Mark Walker's delightful translation of Professor Tolkien's masterpiece, The Hobbit (Hobbitus Ille). I was especially charmed by Tolkien's maps, translated into Latin (with Latin runes!). Mons Solus (the Lonely Mountain), Montes Nebulosi (the Misty Mountains), and Montes Glauci (the Grey Mountains).
However, I was struck by how different these names felt from those that I am familiar with from selenography: Montes Apenninus (the Apennine Range), Montes Agricola (The Agricola Range), Montes Riphaeus (the Riphean Range), and Mons Agnes (Mount Agnes).
So I am puzzled, I guess I have several questions. I'll try to break them down.
1) I assume that forms like Montes Apenninus are actually appositives. Are there special rules about how to use them?
2) How do Latin speakers use names of mountains and names of mountain ranges? I've never encountered them outside of the Bible (where they are indeclinable) and, now, Hobbitus Ille. I am interested in classical, medieval, early modern, and new Latin.
3) Speaking of The Hobbit (and its more famous sequel), how would you say "Mount Doom"? Mons Fatum, Mons Fatalis, something else?
As I said, I haven't encountered this issue "in the wild" (reading Latin texts), nor in any of my rather large collection of textbooks. Can someone give me some pointers?