Not necessary as a command. "Everyone, do your homework", everyone, go to bed. Everyone, take your medicine.
Why mitz tapuzim and not mitz tapuz? It is bakbuk yain, not bakbuk yanot, after all.
I don't think there is a reason for it. That's just the way it is.
Even closer to home, מיץ לימון and not מיץ לימונים. Like Danny wrote, there seems to no rhyme or reason to it.
Everyone doesn't drink orange juice. Bah.
Why isn't את needed?
Because the object is not definite.
Are you able to make compound constructive states?
Eg. בקבוק מיץ תפוזים? Where would you put any propositions or definite article?
Yes you can. That is covered in the construct state 2 skill.
You'd say בבקבוק מיץ תפוזים. Or definite ה comes in front of the last word. בקבוק מיץ התפוזים.