What you are trying to do is difficult because it is bad programming practice with or without using globals. Basically you are trying to poof variables from your base workspace into you callback workspace.
A better approach would be to load the variables in a controlled manner into the GUI workspace and then pass them into the callback workspace. The loading into the GUI could be through text boxes, a load file box, or even a load data box (where evalin might be both useful and acceptable). Once loaded into the GUI you can share them with the callback like any other data.