[masaegu]

Originally Posted by TBox

Is it just poor writing, or is it not high literary style, or both? And if it's not too difficult a question, what would've been a more literary way to say it, for the comparison?

(It's fan fiction, and someone is enjoying it, because they keep responding to his わっふるわっふる.)

It is both; It is terrible writing. That is why I said what I said to the serious Japanese-learners in my last post even without knowing it was from a fan fic. Trained writers simply do not write like this. It has two rather big issues.

1. 「もう星さんの事はどうでも良くなってて」 is clearly an imaginary statement but the author makes no effort to show it to the readers. He just places a comma after this and continues to describe what he was feeling in his mind at that moment, resulting in misleading some readers into thinking なってて was in the continuative, which is what happened to the native speaker you mentioned in your first post.

なってて is the colloquial form of なっていて but it was okay to use the colloquial form there because it was part of an "imaginary quote".

2. Uses 僕に twice. The second one is completely useless and it makes the sentence grammatically incorrect.

Examples of how a better writer would have written this:
(but I will use as much as possible from the original．Otherwise, it will not serve the purpose of structure comparison.)

この関係にずぶずぶはまっている僕に、『もう星さんの 事はどうでも良くなってて』と、こいつの背中が（そう ）語りか けている気がした。

『もう星さんの事はどうでも良くなってて』と、この関 係にずぶずぶはまっている僕にこいつの背中が（そう） 語りか けている気がした。

こいつの背中が、この関係にずぶずぶはまっている僕に 、『もう星さんの事はどうでも良くなって』と、語りか けている気がした。

This fan fic writer failed to use the key word 「と」.

Where did the pray come from? Is it because the よくなってて is in the te form? I thought it was just a continuative, but I can see it being an imperative now.
And now that we have the complete sentence, am I right in that the に on the first 僕 with a clause pairs with the 気がする, and the に on the second 僕 with the 語りかけている?

It is because the form 「～～ていて」　is often used in making wishes. It is short for 「～～ていてほしい」 or 「～～ていてください」. Another very common form is　「～～ように」 .

As for your second question, the second 僕に should not be there as I stated earlier. The remaining　僕に modifies 語りかけている.　The subject for the verb 気がする is omitted. It is, of course, the narrator.