开心 can be either joyful, delighted or the mood of happiness.
快乐 is the mood of happiness. The dictionary defines it as:
So, we say 我今天玩得很开心，not 我今天玩得很快乐.
But they can be synonyms as in 和他在一起很开心/快乐. In this case, both denote the sense of the mood of happiness.
Characters with "mouth radicals" are often chosen for transliterations. This is especially helpful to show that it is a proper noun rather than anything else, otherwise the reader might try and put meaning into characters that are simply there for their associated sounds.
A similar concept can be seen in Chinese onomatopoeia. For instance "choo-choo" the ...
In your case, both 开心 and 快乐 are used as adjectives, which denote “happy”. Translated, the sentence means “I am happy when you are happy.”
However, only 快乐 can also be used as a noun.
This would mean “he obtains happiness from doing good deeds”. It would sound weird if you substituted 快乐 with 开心, which would then make it “he obtains happy ...
事 refers to 事情(affair, matter, thing, etc). 什么事情 is specific - what thing. 发生了什么 is generic. They can be interchangeable in most of context, in which we take 发生了什么 as the shorthand for 发生了什么事.
However, 发生了什么 could also be shorthand for 发生了什么现象，发生了什么反应，发生了什么变化，etc.
E.g. 这两种物质混在一起会发生什么？== 这两种物质混在一起会发生什么现象/变化？
In the case above, 这两种物质混在一起会发生什么事 ...
When you start the first part of a sentence with a 虽然 (although), the second part would always start with a "but" (但/卻)
Although these are two different plants, they are both in the same orchid family [after all].
(Both would suffer from disease that only infect orchid plants)
I think they can be used interchangeably. When it used in songs or poems, rhythm is more important than which word to be used. Sometimes, to avoid using the same word in one sentence, people tend to use a different one too.
Well, what does a big belly do? It sticks out, protrudes!
Have a look here, definition 2. especially: 凸出
The snowman, with his big belly sticking out, playfully said, "I am winter."
(There is a German joke about big-bellied men, this just reminded me of it, but you'd have to know German to understand it: Er ist ein ...
This sentence reminds us of the fact that I'm also a language hobbyist(assuming I'm well known as a 语言的从教者).
With 既, it parallels the two roles (语言的从教者 and 语言学习的爱好者), which share the same level of importance.