If you look in the lil instruction book with the game, it says druids cap at lvl 14 (which bothered quite a bit, but my wife still wanted her druid for multiplay).

Now I noticed in game, that at lvl 14 you can take yer druid to the 'druids grove/circle' and challange your right to become 'The Druid'.

Has any one done this? Can you become the head druid and gte lvl 15? or was this not added fully?

In reference to your Druid question, Yes Druids can only reach 14th lvl. The XP cap of 2.95m is set to stop them from reaching 15th. According to 2nd Ed. AD&D rule you have to defeat a druid at 12-14 lvl to advance. BG2 is just trying to include the great druid challenge as a RP element. If you win you become great druid (14th) of the area. It's the class quest for Druid. The game never allow you to be the grand Druid (15th).

However, there's a very strong chance of that being added in the inevitable expansion to BG II...

Does each class have a "class quest" like the Druid, or is the duel for the Druid grove that class' equivalent of a Stronghold?

I'd say its the class equivalent of a stronghold. My paladin never got a stronghold either; how could he, anyway, he is a paladin :). He did however get accepted on the Order of the Radiant Heard, and had to run some errands for them.