The illiterate fan fic writer has me on the wall again.

今、ナズを見失うと、二度と心通わないような気がした 。

I want to translate this as "If I lose sight of Nazu, I felt my heart would never waver again," except the story's more or less made it clear that his heart is never going to waver no matter what happens next. And he's mixing his tenses. And he never loses sight of her, if it matters.

Then there's this one: It's a girl talking to a guy. She loves him. He loves someone else, and tells her about it. This is her summarizing what came after.

そして君の告白を聞いてもう無理だった。 気がつくと君を誘惑した。 幸せだった。ずっと君の傍に居たいと思っていたから。 その幸せを手放す事は出来なかった。 だからあんな約束をした

Two things:
1. The 気がつくと is a different tense than the rest of it, and
2. what she describes comes out of order. He confesses, they make a promise together, and then she seduces him. But she describes it as he confesses, she seduces him, and the result is why she made the promise.

I want to believe that there's a secret to the と particle that unravels everything for both of these sentences. Otherwise, I'm lost.

「今、ナズを見失うと、二度と心通わないような気がし た 。」 is actually a good sentence. The author is a poor writer as I stated clearly last time but he could not be writing every sentence terribly. It may look to you as if he were mixing tenses but he is not. The reason that you feel he is is that you are translating word for word into a language that is not even remotely related to Japanese. 　

Perhaps the word 「今」 got you confused. Unlearn "今 = now"　now. 今 refers to the moment in the past when he felt this way, not the very moment that he is writing about it. Let me give you the best free piece of advice that I have given in weeks here. Whenever you feel like you are seeing a tense mix-up in Japanese, pretend there is a hidden direct quote in the sentence.

Pretend this is what you are seeing:
『今、ナズを見失うと、二度と心通わない』ような気が した 。

If your Japanese is good enough to be reading a fan fic, I am certain that you can see exactly what I am trying to say here.

BTW, 二度と心通わない does NOT mean what you said. It means "we will not be able to understand and love each other again." This is Reason #2 that prevented you from comprehending this sentence.
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Before attempting to answer your question. I must ask this. Does it actually say 「気がつくと君を誘惑した。」? It is not a natural-sounding phrase. 「気が付くと」 should be followed by 「していた」.

1. There is no tense problem with 気がつくと. None. Give up super-direct translation.

2. It does not say that making the promise was the last thing she did.

「と」　is used for a different meaning in each sentence.
今、ナズを見失うと = if
気がつくと = before I knew it

Yes, yes I do. Thank you.

The only reason actually. The whole sentence fits a pattern I'm used to when I know the right meaning. Funny that.
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You're right. It's していた. I edited out the parts I'm too embarrassed to admit to reading and got too aggressive. It was originally して and then... some explicit bits... ¬.¬

Is that an idiom or is there a pattern I don't know?

PS: I'm an utter fake. I have two semesters of Japanese, a dictionary, a grammar, the Tanaka corpus for reference, and a lot of patience. The illustrations on this story generated a lot of WTF among my friends and I'm still the one with the most Japanese skill, so I'm slugging through it. The pictures were so innocent, the story is so :o

Thank you for your help, as usual.

No, it's just a more natural way to express the literal meaning of the sentence in English. If we take a look at the phrases「ナズを見失うと、二度と…」 and 「気がつくと君を誘惑していた」 we find no crucial difference in Japanese.
Also recall that is more a "when"-if than a "conditional-if."

気がつくと君を誘惑していた => I come to my senses and I was seducing you. => When I came to my senses, I... => Before I knew it, I...

How old do I sound like?
 

私の声は何才のように聞こえますか?

is this right?

You don’t need that "のように". If anything, you can exchange it for "ぐらい"。

One more then?
行動に移さないご主人様が悪いと自己正当して、ご主人 様も傷つけたんだ。
I have some sort of mental block with that と particle. No matter how many explanations of it I read it fills me with dread when I see it, even though it should be very simple.

Start with the simplest phrases, then. Do you have trouble understanding these sentences? Would you have used 「と」 had it been you writing these? If not what would you have used?

「このピザはおいしいと思う。」
「スミスはハンサムだと思う。」
「『君は本当にきれいだ。』とボクはマリアに言った。」
「自分はバカだと感じている。」

「と」 is used for quoting speech, thoughts, sensations.
「と」 works sort of like, if not exactly like, "that" in phrases like:
"I feel that ~~~."
"Someone said that ~~~."
"You think that ~~."

In the sentence 「行動に移さないご主人様が悪いと自己正当して、ご主 人様も傷つけたんだ。」:

Someone 自己正当した that 行動に移さないご主人様が悪い and the same someone ご主人様も傷つけた.

I'm currently having trouble understanding some sentences.

1. http://i.imgur.com/iLsaU.jpg
I'm thinking of interpreting the line as "But I don't know whether it is because I can't see you as someone for romance or because of another reason", but I'm not really sure. Please correct me if I got it wrong.

2. http://i.imgur.com/jf9SJ.jpg
The word 肩 is bugging me a little. Can I take it that there were cherry blossom petals on Aoi's shoulders, and they fell after she had shouted the line out? Also, what would be a good English equivalent for 素っ頓狂 in this context?

のように→ぐらい『に』

「私の声は何才ぐらいに聞こえますか?」