"You guys are all busy."
很 is generally added before adjectives in affirmative sense. When translating, it can be ignored.
If I'm not mistaken, when it comes to emotions, states of being, etc. then 很 is more appropriate than 是.
是 is (generally) more for things like being a doctor, or student e.g. 我是学生
Versus something like 我很高兴, which here simply means "I am happy" rather than "I am very happy". I think this confuses a lot of people (myself included!).
Yes... but you wouldnt be specifying that much... if you want to be very spesific you use 都