1. Plosive meets slithery thing (we hear) on Caradhras.
2. ABBA song swallows little Beatrice. It’s exhausting!
ORCS: Oo, they’re awful!
NEEKERBREEKERS: Cousins of Vogons? They torment with onomatopoeia!
WATCHER IN THE WATER: Submerged stalker, or just a wet TV addict?
EMYN MUIL: Lovable rugby player loses surname and liquid, but gains an equine hybrid here, we hear!
AMON HEN: Grumble about chicken? Here?
THIRST: ‘Lotsss of thisss in Mordor, my Preciouss!’ (he gasped).
HUNGER: Do this only for a taste of justice or a world of truth. (The twin of the last clue).
10. Nice place? No, a slum. Messed up.
NAZGUL: Breathing apparatus swallows map-book, in confusion. (They’re so bad at finding people, you’d think they had, too).
HORROR OF SHELOB: Terror of Cirith Ungol.
AMON AMARTH: Morning marathon around (or rather up) here – and the final one.
14. To hear these might well dement. Perhaps they were played to Frodo in Cirith Ungol?
TREE-ROOTS: Much harder than feather-beds!
HOMELESSNESS: Hateful road brings epiphany of danger – and this.
ARAGORN'S SINGING: Beren and Luthien, brought to the camp-fire by this.
DEAD MARSHES: Inanimate bogs.
You are very close regarding the theme, and right about the connection with Frodo. (So number 1 can't be Balrog, because only Gandalf had to deal with him on Caradhras itself).
Watcher in the Water is correct.
The slithery creature often has legs, but not always. (Some kinds don't. I 've looked it up).
Bizarrely cruel? Moi?
No, I just put in a few jokey, light-hearted ones, to stop the puzzle being too maudlin.
Bombadil was a good guess, but I am referring to songs I personally find hilarious, and which have afforded me some of my most enjoyable moments in Tolkien fandom. But some people can't bear to hear them.