Yes the formal plural (and formal singular) is U. But is considetred old fashioned and you wouldnt usually hear it in conversation. (The formal singular however is, when you talk to strangers or people older than you it is the polite pronoun to use) Only in official speeches you might hear it (the U as formal plural).
Not necessarily. In general they are interchangeable and it does not have mean that if you use jij/wij/zij you are emphasizing the subject. However, when you do want to emphasize the subject you will only use those and not je/we/ze.
On a side note, hij is always hij there is no he.
He isn't wrong though. U is indeed the formal formal. But when writing letters or an email or things not everyday speech you do often use wij/jij/zij. We/je/ze are the more eroded forms like ya for you in english. You wouldn't use ya on an announcement would you? Because it doesn't look "proper". Difference is ya (afaik) is considered slang and not part of the official language but je etc is. But there is still the distinction between everyday speech and more official situations.
I guess it is comparable with contractions in english. We are is more formal than we're both are correct but when writing a letter (to the council, not your buddies) /pamphlet or whatever you use more "high standard" language. Not: We're gonna see what's up. But: We are going to see what is going on.
Californian and interested too. Seems like the U.S. is divided in two when it comes to the you plural - http://www.dailymail.co.uk/news/article-2336660/Yall-you-guys-Dialect-maps-showcase-Americas-linguistic-divides.html
De definitely is and should be audible (atleast on the woman's voice I got).
If a word ends in -en the last n is often hardly pronounced. It is acceptable if you do or don't, usually it is done somewhere in between (very soft/trailing of). This goes for verbs ánd nouns. So fietsen the verb (riding a bicycle) and fietsen the plural noun (bicycles).