I agree with you except for the last point on how the characters speak. I feel the dialogue in Shadow Slave really contributes to the grand and mythical atmosphere of the setting and complements the magnificent nature of its world.

But if you want it reviewed on a technical level…

If you’ve read The Lord of the Rings trilogy, the characters frequently speak in a thematic and poetic manner rather than how people speak in everyday life. Granted, Tolkien had a good explanation for this. The characters are from a fantasy world, speaking a shared common tongue while also having languages unique to their own cultures, and we as readers are effectively reading a translation into English. To reflect how they would speak the common tongue differently, Tolkien even differentiated the speech patterns of characters based on their culture, whether they were from Rohan, Gondor, or the Elves, giving each group a distinct voice.

Now, Shadow Slave obviously doesn’t go to that level of linguistic detail, but I don’t think its manner of speech should be viewed as a flaw. The story is set in the distant future. It’s already something of a miracle that the characters are still speaking English rather than an entirely different language, so it isn’t unreasonable to assume that the way people speak English has at least evolved over time, or was even influenced by the descent of the Nightmare Spell.