How post-modern of everyone.

Keep to the tone of the rest of the novel. And make sure everyone acts within character. If the "betrayal" requires a character acting in a way completely against his/her nature, with no previous provocation and no foreshadowing for the reader, the reader will indeed not believe it and be very upset at you, the author. If you keep everyone acting true to their nature, the ending may well write itself.
P.S - Duke Sandre from Tigana by Guy Gavriel Kay?
[This message has been edited by cassandra (edited April 19, 2001).]