Q: what does "they upended their hearts" mean in here?

they upended their hearts and emptied out every last
drop of love they had ever felt for her とはどういう意味ですか?
A: upend = completely empty out a container by flipping it over

They expressed all their love for her. It is as if their hearts were buckets full of love and they emptied the buckets to pour out all the love.
Q: “Insert hearts and flowers” とはどういう意味ですか?
A: “Inserts something” is a cute way to express some emotion. For example, “My boss gave me a promotion today. Inserts happy face. Thanks boss!” In this case, “inserts happy face” means, “imagine me with a happy face on.” In her writing in your example, she means “imagine me with 💕 and 🌺”
Q: you don't collect hearts とはどういう意味ですか?
A: This idiom is applied "emotionally" so whether or not it's in the heart of mentally.. It can be used.
Q: eat them to our hearts' content とはどういう意味ですか?
A: It means to eat however much of something that you want to eat. Eating a food until you are content
Q: The more far apart your hearts are, the louder you have to get your voice heard. とはどういう意味ですか?
A: hmmm I'm not quite sure. Grammatically it should probably be, "The farther apart your hearts are, the louder you have to speak / be to get your voice heard."

It could mean that the more different your opinions / feelings / thoughts are, the more you have to do to convince the other person to agree with you.


Q: He will remain in our hearts と He will stay in our hearts はどう違いますか?
A: The meaning of the sentences is both the same. Remain its just a fancier way to say it.


Q: Or simply fill our hearts with tenderness.what does “simply “mean in this sentence は 英語 (アメリカ) で何と言いますか?
A: thank you so much
Q: Get well soon Lebanon. our hearts beirut with you は 英語 (イギリス) で何と言いますか?
A: "Get well soon Lebanon. Our hearts are with you, Beirut!
Q: hearts of iron は 英語 (アメリカ) で何と言いますか?
A: cold hearted


Q: What does 'collecting hearts' mean??
A: Heartbreaker person
Q: Not only our hearts get hurt by people at times, but also we get healing by another people. この表現は自然ですか?
A: Your sentence has. some grammatical mistakes as well. I would say it something like this:
Although at times, our hearts get hurt by other people, they may also be healed by others.
Q: Those hearts look like butts.の発音を音声で教えてください。
A: QAの全文をご確認ください
Q: to live in the hearts of those we leave behind is never to have died whatsoever. この表現は自然ですか?
A: It isn't grammatically wrong, just feels slightly unnatural in the sense of, it's a poetic phrase and 'whatsoever' feels out of place for reasons I can't really put my finger on.

"At all" feels more poetic, basically. But no, it is not grammatically wrong.
Q: Which one of the two hearts more heavy? この表現は自然ですか?
A: Maybe try, ''Which one of the two hearts is heavier.'' or "Which of the two hearts weighs more?'' or ''Which of the two hearts weighs the most?'' or ''Which heart will out weigh the other?'' or ''Which heart do you think weighs more.'' or ''Which heart do you think is the heaviest?'' or Which of the two hearts do you think weighs the most?'' Just some ideas, hope it helped. Good luck!