To delete folder from p2k filesystem i use this:
Simply move cursor on the folder and delete it.
Folder and its contents will be gone (at least on my e398)
If not, there are a little trick:
-Set both panels (left and right) to p2k filesystem
-on the left the folder list, place cursor onto the folder
-in the right open desired folder, select all files , and delete them
-goto left, and delet folder which is empty now
invisible folders: p2k filesystem will not show empty folders

To make it visible simply re-create it (if you remember its name)
When P2kc create a new folder , a little file put there (to make non-empty the folder)
When you re-create invisible folder, actually the folder creation can fail, but a little file will put there.
If you have V421 you can copy a file directly an invisible folder if you alter the destination path before copying
Good luck