I need help with this translating this paragraph

Question

혹시 기분이라도 나쁜 건가 라는 생각이 절로 들 정도로 표정이나 어투가 딱딱한 것이다. 덕분에 혹시 저 누나는 로봇이 아닐까? 하고 어린 마음에 고민헸던 기억이 있다.

혹시 기분이라도 나쁜 건가 라는 생각이 절로 들 정도로 표정이나 어투가 딱딱한 것이다.
This is incredibly confusing. Can't even begin to understand what it's trying to say. I've read up on the grammar points and for some of them I thought I had a good handle on like (이)라는 and 이나, but the rest of the sentence is throwing me off. Especially 들, who is just sitting there and seemingly not serving any purpose, threw me for a loop. I'm guessing it's from the verb 듣다 since this is talking about the sound of her voice (I think) but that's as far as my understanding goes. It could be that the writer has omitted/contracted something that I'm completely unaware of.

덕분에 혹시 저 누나는 로봇이 아닐까?
"Because of that maybe she is not a robot?"
Maybe my understanding of 을까 is shallow but this seems like an almost obvious translation. Also not too sure about 덕분에, but there seems to be no possible way it could mean anything else than "Because of that"

하고 어린 마음에 고민헸던 기억이 있다.
"And in my child's heart there was an agonizing memory."
I'm not too sure about this one either. All examples of 하고 I have found on the Internet don't include it at the beginning of a sentence and since I only just learned about 던 "agonizing" might not fit best in this sentence either.

Answer 1

혹시 기분이라도 나쁜 건가 라는 생각이 절로 들 정도로 표정이나 어투가 딱딱한 것이다.
= The thing is, her facial expressions and speech are stiff to the point you'd jump to the thought she might be in bad mood.

들 is not from 듣다 but 들다. 들다 is an abstract version of getting in or into something, the 들어 in words like 들어가다 and 들어오다. It is used for more oblique cases of getting in, like "turning in" for the night (잠자리에 들다), figuring in the ranking (순위에 들다), getting into one's liking (마음에 들다) etc. where 들어가다/들어오다 might not sound natural.

Here, 생각이 들다 means a thought comes to mind. It's the standard expression for thinking about something, but it implies you are led to the thought instead of actively thinking of it on your own. The form 들 is 들(다) + -(으)ㄹ (just like 이었을 in your previous post). 들을 becomes 들 fusing ㄹ/을 into single ㄹ for phonetic reasons.

• [[혹시 기분이라도 나쁜 건가] 라는 생각이 절로 들] 정도로 = to the point/extent [that the thought that [she might be in bad mood] springs to your mind (절로 들다)].

VERB + -(으)ㄹ 정도로 is the standard expression for "to the point/extent that you VERB/ are ADJECTIVE".

-(으)ㄴ 것이다 is a common form of embedding a clause to create a two level structure for various effects. It defies simple explanation, but in this example I think it's similar to "It is that ..." or "The thing is that ..." in English. It introduces indirectness so that the reader thinks of what's being said more in light of the surrounding context.

덕분에 혹시 저 누나는 로봇이 아닐까? 하고 어린 마음에 고민했던 기억이 있다.
= I remember grappling with the thought "Could she possibly be a robot?" in my young mind because of that.

You might be taking it as two sentence but it's actually one. The question mark is just part of the embedded cluase. It is really not required because the -ㄹ까 makes it more than clear enough it's a questioning thought, but you might include it for emphasis and other reasons.

라고 in your other sentence and 하고 in this one play the same role of quoting a phrase. It can be dropped or phrased differently depending on how direct or indirect you want to make the quoting.

For example, all these are possible.

1. "혹시 저 누나는 로봇이 아닐까?" 하고/라고 생각했다 (direct quote).
2. 혹시 저 누나는 로봇이 아닐까(?) 하고/라고 생각했다 (informal mix of direct and indirect quote).
3. 혹시 저 누나는 로봇이 아닐까 생각했다 (compressed version of #2).
4. 혹시 저 누나는 로봇이 아닐까 (하고) 의심했다 (with a more specific verb 의심했다 (suspected).
5. 혹시 저 누나는 로봇이 아닌지 궁금했다 (with -지 궁금하다 (wonder if/whether) - can't use 라고/하고 since 지 plays a similar role).