On would/wouldn't:
One has to look at the meaning of the original sentence, which is accurate, as @Emilia42 originally wrote it. I went to an English grammar school, by the way.
"Just because you wouldn't do that doesn't mean others wouldn't."
You wouldn't do it.
Would others do it ?
Yes, others would do it.
No, others wouldn't do it.
Now, you connect the two thoughts together.
You wouldn't do it, but others would do it. (people are going to behave differently)
You wouldn't do it, and also others wouldn't do it. (people are going to behave the same)
So then:
"You wouldn't do it doesn't mean others wouldn't do it. " You are saying you wouldn't do it, BUT what you decide to do does not change someone else's decision, and they may do it, or not.
It's not about how many negatives - it's about the original meaning of the sentence. She is saying just because she would not do something does not impact the decision of another person, who may do it, or not. Put another way, she wouldn't do it but another person might behave differently.
"Couldn't care less" is correct.