Not really, 8 March is definitely not any kind of official start of Jubilee celebrations.
Technically, the Jubille celebrations have already started a month ago since Her Majesty ascended to the Throne on 6 February. There have already been fair few events dedicated to Queen Elizabeth's Diamond Jubilee. However, the majority of celebrations will take place in June, probably to coincide with the anniversary of the Queen's Coronation ceremony, which took place on 2 June 1953.
There is no specific reason why Camilla was not included in this particular engagement. It was announced via the royal diary that this event will be attended by the Queen, the Duke of Edinburgh and the Duchess of Cambridge. The previous event was attended by all three royal ladies. In coming days, Kate will join several senior royals during official engagements, including one with Prince Charles and the Duchess of Cornwall.