①Slurpは？

I feel noodles taste better when I slurp them.

のように、slurp/slurp（すする）が一般的だと思いますが、他にも

I wonder if pasta tastes even better if you slurp it up making a lot of noise!

のように、make noise とか make sound （音を立てる）でもよいかと思います。

②hold the bowlは？

You shouldn't hold the plate up!
Also, I don't know which plates I can hold up and which ones I shouldn't.

のように、hold up（持ち上げる）ですね。

ただ持つだけならholdでいいのですが、口に近づけるためにテーブルから浮かせるのでupですね。

③Japanese sitting postureは？

正座（せいざ）といいます。
Japanese sitting posture（日本流の座り姿勢）
Sit on one's heels（かかとの上に座る）
Seiza（正座）
などいろいろな言い方があるようです。

動画の中ではこのようにしました。
My parents trained me to sit beautifully and straight when I was little.

sit beautifully and straightとし、正座が周りの人から見て美しく凛としている様子を表現しました。

Should you leave the food that was served?

I think eating all the food you're served is common etiquette in Japan.

It is a sign of gratitude to the creator and a wasteful spirit.

However, in some countries, it is considered rude to eat everything.

In this day and age where food waste is becoming a problem, I think the Japanese style of "Eating everything" is in keeping with the times. 

 Am I the only one who thinks this is more of an environmental issue than good etiquette?