King? The Great Link by its very nature has no ruler. The Female Shapeshifter was not any more powerful or important than any other Founder. (Nor was she a female for that matter.) By rejoining the Great Link, Odo was becoming an equal member in a community, not an authority figure.
As for reshaping the Dominion-- It's doubtful. I believe that the Founders will honor their treaty, as it's the only way to ensure that Odo will remain. However, convincing the Founders to keep the peace with the Alpha Quadrant (and perhaps the Dominon's other neighbors as well) and convincing them to reshape the Dominion are two very different animals. Given the slow methodical nature of the Changelings, it will likely take him centuries of hard work to reshape the Dominion. Perhaps in time others of the Hundred will return with positive experiences amongst the solids and aid his cause.