1. "бы" means "would", like when you are projecting or planning but not 100% sure and talk more about wishes than real actions. The whole thing could be translated like "I wouldn't want to change anything" (or something like that).
"стала" means "an attempt", here it's not the beginning of the action, it's the whole concept of if I do something or not do.
2. "я бы ничего не меняла" sounds almost the same, correct, with just a tiny difference like "I wouldn't change anything" vs "I wouldn't want/dare to change anything".
3. Водоем is any natural water surface (Sea, River, Lake, Pond)... except for a swimming pool I guess.
4. Допоздна = till late, на продлёнках: продлёнка (fem) means "продлённая группа" (prolonged class) when children whose parents can't take their children from kindergarten in normal time because of job or something, they all are gathered in one room and stay there with one of the teachers till their parents come to pick them.
5. no, not people. Don't overcomplicate things, it's just about multiple facts of getting acquaintant with people.