Though I think he would have opted to reign under the name George, the man who would become Belgium's first king in OTL was offered previously, and passed on the throne that went to Otto of Bavaria. Suppose Leopold had become King of Greece, does he fare better than Otto did, and what becomes of Belgium under these circumstances?
Theres a timeline called pride goes before the fall about greece. This is one of the core points of the story. You should check it out, you would probably like it.