Unfortunately you asked two questions - I am only allowed to respond to one. I have chosen to focus on the climax of this amazing novel. Let us remember that the climax is defined as the high point of the plot, the story's most exciting or suspenseful moment, when something happens that decides that outcome of the conflict.
Certainly then, for me, this occurs as the Waknuk force and the Fringes people clash and at at the same time the Sealanders arrive to rescue Petra. This is the big conflict that the novel has been pointing towards and it is the most exciting, as we are concerned for David, Rosalind and Petra and we are worried if the Sealanders will arrive in time and how they will save the main characters.
The falling action then would take place after the Sealanders have arrived and dispatched of the fighters with their cobweb weapon. Releasing the main characters and their final arrival in Sealand form this section of the plot.