I think especially your second option "Don't make trouble where the is no trouble/none" sounds just fine. In case you're interested, here are a few other American expressions for the same idea... "Don't stir up trouble." "Let sleeping dogs lie." "Don't poke the bear." (I don't know whether these would sound normal in British English or not.)

That's a very good question...and a bit difficult to explain. By saying "They discovered that the tree house belongs to...", the author is moving the action into the present. The discovery was made in the past, but the tree house still belongs to Morgan le Fay. It wouldn't be incorrect to say "They discovered that the tree house belonged to...", but it would move the story into the past. (Maybe the tree house used to belong to Morgan le Fay, but no longer does.) I hope that helped at least a little bit...

please help me modify this essay!thanks

My primary school teachers
There were more than a dozen teachers during my time at primary school, but I only remember three of them.
The first one was teacher Sun. She taught us Chinese for two years. She was about forty years old then. As I remember, she had big, bright eyes and black curly hair. She often smiled ,so all the students thought she was a kind teacher. She was extra nice to me, and I have been fond of mandarin ever since then. It is with regret that teacher Sun died because of cancer a dozen years ago. she was a good teacher.
Teacher Zhang was a male teacher in his fifties. He was very strict. When he looked at you, he could be scary, so the students were always afraid of him. I don’t know why he was especially good to me. When I/he left the school, he gave me a dictionary as a present. I treasure it to this day.
Teacher Wu was a happy teacher. She was good at joking. She often played games with us . We could never help laughing at her lessons. Unfortunately, her only son died by drowning a few years ago, but teacher Wu is an optimistic woman . The year before last, when she was sixty years old , she gave birth to twins through in vitro fertilization.
I loved my primary school teachers. They were responsible teachers,  and they provided me with a happy childhood.

A Rainy Day

I was planning to go shopping with 2 friends today, but we changed our mind because of the bad weather. It's been raining the whole day.
I am feeling depressed/blue/sad, just sitting in the living room and doing nothing . One of my family members passed away yesterday evening/last night, She got sick three months ago and her condition never improved. After she had been in a coma for 4 days, the doctor said, it was time to let her go.

I'm sorry to hear about your loss. My condolences.

I agree. "It is a typical American family", but "The Taylors are a typical American family".

"The pressure of modern life is great" sounds fine. No problems!