i still would like to know with a certain how dos Xemans knows about Ventus localization on CO
Well...becasue Eraqus's heart was a part of the new Xehanort too, along with Terra's and MX's. And Eraqus told Aqua where to hide Ven if things got bad, which they did. Thus, Xemnas goes to where the land of departure was, but finds Castle Oblivion instead, and he can't find Ven becasue only Aqua can (she designed the castle that way, as Eraqus asked).