I think it's kinda dangerous to believe translation's all art. It's about trying to get across the meaning of a text accurately, the art part should be more about how you do that or what you can do to smooth things out. Like 人それぞれ is an official expression meaning "to each his own" or "people have their own ideas", generally the idea of personal preferences, so it's not really up for debate on whether 人 is singular or plural, the inherent expression's plural. And I don't think you need to make it "each and every person" since that kinda reads a bit clumsily, I'd say any general plural would do fine, like "people" or "everyone" or "we all", etc. Also, she doesn't really directly refer to a person with a pronoun like あの, which kinda precludes the idea of a singular reference.
And と思わない does just mean "don't you think so?", a request for affirmation on a statement, like "right?", so they're both interchangeable. Although in context "right?" is more casual so it might not fit.
I think you might be ignoring the quoting Mari does with 幸せって, it should be clear that she's linking the expression 人それぞれ with happiness, 幸せ, with って, linking the expression of personal preference with happiness, ergo, personal idea of happiness, which makes sense in context since she goes on to say what makes her happy.
You're free to ignore all this, this is just my own approach to translation and you've already changed it, just wanted to say that it doesn't hurt to look a bit deeper into the grammar and wording of dialogue and consider how natural the ways you can translate it sound.