I don't have 300GB of games on my internal to check if it really causes such delay.
Normally multiMAN should show its first message about 10-15 seconds after you launch it. It loads a lot of images, loads a lot of libraries and clears files. It is not a simple program which loads instantly - sorry.
The overscan for the filemanager is only for both pane lists. You may still not see the TOP and the BOTTOM lines, but it is enough to perform all the operations (copy/rename/move/delete).
If you get the black screen - just wait for it to complete. If you have nethosts misconfigured it may cause the delay. Scanning a lot of folders also may take time.