The US Open is (as Ernie Els has said) the de facto world championship of golf. It is not a track for the average or even very good golfer. I don't happen to agree with Throatybeard about the quality of the course and the tournament. It is intended as a supreme test of golf ability. Especially accuracy. And also course management (decision making). And play on and around the greens.
While the Masters is a tournament for long hitters (think Snead, Palmer, Nicklaus, Tiger, Phil), it is also a test. Tom Watson (I think) said that, if one dropped the ball at the 100-yard marker on every hole, a 15-handicap golfer playing Augusta for the first time would not break 100. Of course, I'd sure like the opportunity to try.
WRT your last question, I believe the course rough and the Sunday pin positions are kept for a day or two afterwards. Members are allowed to play the course, of course, and maybe even some sponsors (depending on the contract with the private club). I did get to play Robert Trent Jones outside DC the day after the first President's Cup, which was fun, although the rough was not US Open style.
sagegrouse