Allemaal and iedereen can both be used here, though iedereen is better (every+one=ieder+een). Compare this: Hi, all! = Hallo allemaal! NB Allemaal and iedereen are not interchangeable, e.g. the sentence Everyone was there can only be translated using iedereen: Iedereen was er.
No, nieuws doesn't have a plural: http://www.vandale.nl/gratis-woordenboek/betekenis/nederlands/nieuws
I didn't know that. Btw, I'm not in favour of including this kind of joke in this course, especially since it often involves a sentence that
1) is never used in everyday practice.
2) is grammatically incorrect; artistic expressions don't have to comply with grammatical standards, because of so-called "poetic freedom".