English translation:
for sure
even more so
underlined/bolded text
Structure:
Verb + こそ
Noun + こそ
:
こそ replaces particles を and が
困った時こその友達じゃないか。
Is there something wrong with this sentence? It doesn’t make sense to me.
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Hey 
It is a good sentence, but without context, it might sound strange.
It basically means that friends should help one another in need.
「留守中に犬を預かって欲しいだなんて、あなたに頼んだら迷惑でしょ？」
「何を言っているの。困った時こその友達じゃない」
“Will it be a bother to you if I asked you to take care of my dog when I am away?” (なんて softens expression here)
“What are you saying! Friends help when one is in trouble, don’t they?” こそ puts emphasis on the time of trouble
lit. aren’t friends what is for time of trouble? (not sure how natural it sounds, I am really sleepy 
)
I hope it helps,
Cheers,
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I think what’s throwing me off is that 困った時こそ is modifying 友達. To me that makes it sound like it’s about a single friend (presumably the speaker), not about friends in general. Like “aren’t I a friend (you can count on) when it’s a time of trouble”, or something like that. It’s really awkward to translate. Am I on the right track or is it really a general statement like you said?
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i’m having trouble understanding why “それこそ僕に与えられた使命だと思っています” translates to “I think that is why I was chosen for the errand”. i can’t see where it’s specifying reason, to me it just looks like “i think that is precisely the errand i was given”
It’s right at the beginning on それこそ!
But I agree its a bit tricky, since its contextual. Remember that それ can refer to something that has already been mentioned in the conversation to avoid repetition.
If you think about what probably could’ve been said before, it makes more sense.
To provide an example (but it could be anything really):
“部長は僕を信用しています。”
“My boss believes in me.”“それこそ、僕に与えられた使命だと思っています。”
“I think that [the fact that my boss believes in me] is why I was chosen for this errand.”
Hope this helps!
In the example sentence それでこそ僕の選んだ妻だ, why would それでさえ僕の選んだ妻だ not be correct?
Isn’t the meaning a bit different?
それでこそ - That’s exactly why she’s who I chose for my wife.
それでさえ - But even so, she’s who I chose for my wife.
I can understand kind of how this meaning comes about, but it doesn’t make sense to me with the rest of the grammar.
困った時こそのは友達じゃないか。Would make a lot more sense but こその友達 is qualifying 友達 in this sentence.
If that’s not the case, it would be helpful to have a more in depth explanation for this one.
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Please don’t quote me on this, but it seems that の in こその友達 is more for emphasis, and makes it sound more dramatic. There are 2 similar examples in the linked Maggie sensei post, also without a specific explanation for の.
Since こそ is a particle, not a noun, I don’t think it can modify the following noun using の the same way as a noun would.
I’m not sure where you got the idea that particles can’t be used before の. There’s even a grammar point for particles + の on bunpro. Although I think こそ isn’t listed.
I don’t think the の is for emphasis as such. It describes the kind of friends that they are.
I didn’t say “particles cannot be used before の”, clearly a particle is used before の right here.
So what you are suggesting is that the sentence is about a specific friend (or group) qualified as “that friend that is for troubled times”, not an observation about friends in general? Like what Seanblue above said.