From the last time I was with some armoured infantry on exercise, the Warrior commander is also the Section commander and dismounts with the Section. This is not a 'waste of his training' as he is first and foremost an infantry leader, and needs to be with his men and fighting the battle. The gunner, I believe, then becomes the vehicle commander. The gunner and driver are also both infantrymen first and foremost, with additional training on the vehicle, but as has been said, don't get out unless they really have to.
As for the Section organisation, two 4 man fireteams each equipped with a rifle, underslung grenade launcher, light machine gun and lsw make up the Section. The gun group has disappeared, but the 'golf bag' scale of weapons means that one can easily be created from the Section assets to meet needs.