To make the sign/encrypt dialog clearer and more foolproof:

Make use of horizontal lines to separate the areas for signature and encryption. Probably add subheadings (maybe with an added icon padlock or pencil). Which has the advantage of saving text, too, like:

Encrypt 

  for me: 
  for others:

Re-introduce the checkbox for "encrypt to others" and place the checkboxes again before the dropdown fields.
All checkboxes are checked by default if the dialog is started via "sign/encrypt".

If you leave them all checked and click sign/encrypt without choosing an other recipient, an error window will come up with a message "Please choose at least one other recipient or deselect encryption for others." and the only option "OK" which closes the error dialog.