Jeremy became concerned about too great a turnout of riders in response to lots of publicity a couple of years back. I recall he gave one weeks notice that year. Since that time, I've backed off myself. The first notice of this year's event went up on here a couple of weeks ago,
http://fksa.org/showthread.php?t=7712
If you are really interested in Florida kiting events and possible timing, you should look over the coverage of past events including the last five years of this contest at:
http://fksa.org/forumdisplay.php?f=115
Unless they've put in meters in the last year, I recall there is no fee for parking. No one charges admission either. You sound like you might be new to the area. I wouldn't expect ANY flat water with useable wind on Saturday. There will likely be a nasty shore break for new kiters although competitors will likely enjoy it. Waves will likely come down a bit on Sunday by the current forecast. If you are not used to kiting in waves, good sized ones at that, I wouldn't plan of kiting on Saturday in that area. Checkout the wave photos from some of the past events here. Flat water is largely unheard of in normal winds in the open ocean, Biscayne Bay and Keys excepted.
There is no legal camping on the beach in Jupiter/Juno or anywhere else on a beach in SE Florida that comes to mind. A couple of the State parks allow it that are near the beach along with the normal commercial camp grounds usually inland. Do your research to avoid problems. Some info along these lines appears in the Florida Guide at:
http://fksa.org/showthread.php?t=2278
Yes, offshore winds are predicted for Friday unsuitable for kiting. Winds may bend sideshore in Jupiter, MAYBE, by micrometeo effects but I wouldn't count on it. Neil has standup paddle surfing events laid on which should work out fine regardless of wind direction.
There is a ton of info on this site, covering all your questions and a great deal more. Look around, you have an inquisitive nature, you should find lots of interest. Wx info for all areas of the state appear in the various quadrant forums, for this area at:
http://fksa.org/showthread.php?t=6734