Why does Juan fear for Mariana's safety even in
"that hidden Paris neighborhood"?

Incomplete question. However, I infer you are referring to the story "The Censors".

Likely because of his background.

Explanation:

More likely his background affected his concern for Mariana. Probably be had experienced government censorship of information where he lived, and so thought it was in such neighborhood that Mariana lived.

However, Mariana was in a hidden Paris neighbourhood likely different from Juan's own.

fears about being blown up

1. this would be first person. it would be like "i walked home from school today" they are the narrator of the story and is a character as well.

2. first person because they used the word "i" in the quote.

3. i'm really not sure about this one i'm really sorry i never read it so i wouldn't know how to answer it. 

4. it would be character vs. society because everyone he meets there is always a conflict of some sort because he's stuck in his daydream.

5. it would be sadness at the loss of youth because the title itself tells us it's about age so it couldn't be about regret. and the tone just sounds sad, not angry. 

i hope this !