It's a great plan!
Then there has to be steps in the turning of the gear.
Maybe half a turn gets Melina through the tear in space and time.
But before she can do the full turn, our heroes stop her
That makes sense to me. The way she turns it affects what it does.
To make this possible, we can alter the existing lore like this so (see the bold text for the change)
The gear is an extremely powerful palm-shaped object that can alter and influence the world. The spirits used it back in the beginning of time to create everything as it is now. For example, Yuuna made light and Kurai darkness. It can be used to alter the world in almost anyway. The only thing the gear cannot affect is the most basic element of existance: Ether. It can only use Ether to create bigger elements through combination. It can also be used for fast destruction.
In the early days of humanity, it was discovered and used by the leader of a tribe to destroy their enemies. The leader did not stop there and sought to conquer everything—until the spirits stepped in and fought to seize the artefact. The effort to do so resulted in a massive battle that resulted in mass death, suffering and destruction. This day, known in history as "The Calamity", was also the day spirits went to hiding due to dissapointment and fear. Yuuna in particular felt shame for her creation.
To prevent another such occurance, they split the gear into seven fragments and scattered them across the country in hopes that something like this would never happen again. The only reason they succeed in recovering it was because humans were not yet fully capable of harnessing is power.
The user must drive the gear in order to harness its capability. Spirits can do it naturally, but humans cannot. They require immense Ether reserves to use it meaningfully.
It works with turns. The way a user turns the gear affects what it does. A full turn will cause the rebirth of the universe.
This makes perfect sense. Melina, as the Goddess, would want to re-create the universe in a way that she likes after she gets rid of God. If that sounds good to you, I can make this edit.