Eu suspeito que este seja o romance de 1999 Todos Instantâneo por Robert Garfinkle.
Aqui está a Amazon descrição do romance:
"...a brilliant scientist discovers the Instant, a paradoxical nonplace that is simultaneously all times and no time. Soon contending armies roam the Instant, stirring its waters as they struggle not only to conquer the world but also time itself. But every ripple in the Instant causes entire cultures and timelines to vanish. Even the first humans are threatened with destruction, though that would mean the end of humanity itself."
Eu diria que o seu "momento" é "O Instantâneo". O romance parece marcar todas as caixas da sua consulta.