On Galadriel, you can bet that if she had accepted the Ring, there would have been an outright matriarchy ... but not a good one. (Note that I distinguish between "matriarchy" and "evil/bad"; they are not necessarily the same thing.
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Regarding Melian, you are still dealing with a patriarchal society. Thingol was still the one who made the decisions, usually listening and making use of Melian's wise counsel, but not always; and when he failed to take her counsel, his decisions failed.
I find it very interesting and gratifying how Tolkien consistently included wise women as powerful influences for good within patriarchal societies.