" Le dodici ore prossime" should this also be accepted?
I'm not a native speaker, but for me that sounds very weird. I think prossimo always goes before the noun.
There are though some people that say "la settimana prossima" instead of "la prossima settimana", but that is just a spoken thing.
Is "seguente" not a synonym for "prossimo"?
Not necessarily. Seguente means "that comes after" whereas "prossimo" refers to something in the future. Ex. "Martedì scorso sono andato a Roma. Il giorno seguente sono tornato". "Martedì prossimo vado a Roma. Il giorno seguente tornerò"