Sean,
That section you first mention is a very steep hill and combined with the down-force of the car, is why it takes ages to pull 5th. If you watch the shiftlights, he gets to the end of 5th, thinks about changing to 6th, but then realises that it would bog due to the gradient, as well as the fact that in a few hundred metres he will have to change back down, so he decides to hold 5th (good call, as it doesn't accelerate any further). On the main straight, again it is uphill towards the end of the straight before going into a steep down hill section, so he holds 5th until just before he crests the rise . If you had actually driven the 'Ring, you would know this . The gradients are incredible and you get no appreciation of them without a rear view camera.