holy poop.
massive cut and paste fail.
We didn't even get to the part that's making me tear my hair out.

もう星さんの事はどうでも良くなってて、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僕に語りか けている気がした

My only excuse is that I was on a computer without japanese fonts trying to cut and paste it from a different forum that wasn't getting me any answers. I feel retarded.

We now have a complete sentence (minus a period :) ).

For serious Japanese-learners, I need to state out front that this is a kind of a butchered sentence that you will NOT find in our finer literature. It is something you might find in a light novel or even manga. DO NOT write like this.

The narrator feels that 「こいつの背中」 is telling him 「もう星さんの事はどうでも良くなってて」. In other words, 「そう」 refers to the imaginary statement 「もう星さんの事はどうでも良くなってて」. The imaginary comment means "Let me pray that you are no longer interested in 星さん." No one actually said it. This, I hope, solves 90% of yout trouble already.

"I felt as if this babe's back were saying 'Let me pray that you are no longer interested in 星さん' to me, who is deeply stuck in this relationship."

The "you" = the narrator

If you don't mind, I have question.

How would you say "What is he trying to say?"

For example, an english speaking person is speaking very bad, unintelligible japanese to two japanese people. One of the japanese people turns to the other and asks, "What is he trying to say?"

I'll have an abysmal shot: かれはなんいってにせんとすることか。

Thank you.

Most naturally, we would say:

「何（なに）が言いたいのかな。」 or
「なんて言ってるのかな。」 or
「何（なに）を言おうとしてるのかな。」
「なんて言ってると思う？」
All with a rising intonation at the end.

There will be no "he" because it is so obvious who is being talked about.

How do you even get にせんとすることか? :)

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.)

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 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」.

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.

ありがとうございました、 Masaegu先生。
Muito obrigado, amigo. :D

Wow, that bad? Ignorance is bliss, I've been enjoying it. OTOH, even in my head I re-write it in English style.

If I may continue to impose, can the subject of 気がする ever not be implied? What particle would mark that?

Thanks.

I typed "attempt" into a dictionary and it gave me せんとする and at some point in the past I read that こと makes the verb preceeding it a noun. So, I thought, せんとすること would become "attempting". The に before it... actually I'm not too sure about that one myself, but it was meant to be a preposition of some kind.

Link to this dictionary? 〜とする means "to try to do ~," but I have no idea what せん means.

行こうとする = to try to go
食べようとする = to try to eat
飲もうとする = to try to drink
etc.