This isn't arachnophobia anyway. Phobia implies fear. Morontagh isn't showing fear; he's just being a nasty, prissy little bitch sneering in contempt and disgust at a perfectly innocent small creature. And as you say, when he "has" to fight the giant spooders at the "climax" he isn't afraid then either; he just blithely hacks them to pieces.

Thorn at least "gets over" his claustrophobia

In the cheapest and most offensive way possible. Phobias don't just magically go away when it's convenient to you. Or rather, convenient to the stupid asshole who just led you into seven chapters of even more torture.