Does the "dans mon secteur" mean "in my line of work"? Is this an idiomatic expression?
Yes, to an extent. You could also imagine a situation where an used-car salesman talks to an used-bikes salesman, and one of them says "Dans mon secteur, ...". They have the same line of work in the English sense, but still a different trade sector.
I wonder if the addition of the verb "pouvoir" makes the sentence less natural: "on ne peut jamais être trop prudent".
"On n'est jamais trop prudent" is an idiom. "On ne peut jamais..." does not sound natural at all, although switching it to a more dialectal "On ne saurait jamais..." improves it somewhat.
Don't try to make a direct calque of the English "You can never be too careful", at the very least French would call for a Present Conditional there.
« Vous avez du cran, ça, je le reconnais. »
I suppose that Sentence B makes perfect sense without the word "ça", so I’m not sure what purpose the "ça" serves here.
It's just a fronted topic. "Ça, je le reconnais" is equivalent to "This I'll give you" in English.
In case you were mostly wondering what the "ça" and "le" are doing together in the same sentence, when they're both pronouns referring to the same antecedent, this is just another of co-occurrence of strong and weak pronouns like "Moi, je parle".
While "*Moi parle" and "*je connais lui" aren't grammatical sentences in Contemporary French, there are cases where the strong object pronoun "ça" can outright replace the clitic pronoun "le".
Your sentence carries a contrastive meaning however ("I'll give you this, but there're other qualities I won't say you have"). In this type of sentence:
- The topic is obligatorily preverbal. Je l'ai dejà dis, ça (I have already said this, no comment on anything else) >< Ça, je l'ai déjà dit (while there are things I might have kept quiet, I have said this one thing)
- Doubling with a weak pronoun is nigh-obligatory (In literary French, sentences like "À mon frère, j'en ai déjà parlé" are possible, but you'd strongly expect "Je lui en ai" in spoken colloquial)
- If the fronted complement usually takes a preposition, it keeps it when it's contrastive topic, but it's usually omitted with a normal topic. Compare "Ta soeur, je lui mentirai jamais" and "À toi, je t'ai ptêt dis des couilles, mais à ta soeur, je lui aurais jamais menti"