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I believe so; there is a de jure New Valyria, and if you conquer enough land I don't see why you wouldn't be able to the title. And as for the second question, I don't believe so without resorting to the console or something really gamey. But the original Valyria wasn't a monarchy anyway. :D
 
You have a law where you can turn a republic in a monarchy if you met the requirements.
 
IIRC you have to become "Triarch for Life" and then there is a decision to turn the republic into a monarchy