Why would buying time work?
Because as a Calamity-Rank beast, Marina's father had certain restrictions. Plus, he could not remain in Labh Salem for long.
As long as Eren kept himself and his allies safe from the dragon's fury, he would have enough time to grow before her father could return to Labh Salem.
Thus, Marina agreed to become Eren's wife after seeing his Immortal Domain in the making. Even though it would take considerable time for him to achieve godhood and fully unlock his Immortal Domain, at least he had the potential.
She could work with him and hide in the deepest fringes of Labh Salem if necessary.
This time, her father wasn't going to stay in Labh Salem for long—his visit was merely to check on the Immortal Artifact he had left in their care. Moreover, he would be preoccupied with preparations for the Witches' Sabbath.
