Complex sentence: The combined entity no longer has power to force Cochrane to stay with her, but Cochrane chooses to stay out of love and gratitude; before departing, the Starfleet trio promise not to reveal Cochrane's presence.
Split into simpler sentences: The combined entity no longer has power to force Cochrane to stay with her, but Cochrane chooses to stay out of love and gratitude. Before departing, the Starfleet trio promise not to reveal Cochrane's presence.