Even after the 2nd time you visit the grand staff you still will be able to visit anywhere you want on the world map (once you have completed the zone). The Tower of Mirrors is the Point of no return.
Any items that you missed after your 2nd time here can be bought at the Auction House
Grand Staff is a magic research facility located in the Sea of Baus being built by Gongora. After the events at the beginning of the game, in which a meteor falls from the sky killing thousands of soldiers, its construction is halted by the Council. It is discovered later that it has been leaking magic energy due to parasitic bugs that were feeding off its magic energy. It may or may not have been the cause for the meteor; it could have just been the conduit that Gongora used to summon enough power to call the meteor down. Kaim, Seth, and Jansen are sent there on their first mission to investigate the project.