Safe to delete the pictures from the camera roll, without them being deleted from iCloud drive?
iCloud Drive is just a way to sync photos and other files between all your iOS devices and computers (both Mac and PC). iCloud does not function as a standalone Cloud storage service, hence if you delete photos from your iPhone they will also be deleted from iCloud Drive. 

You can however upload your photos to other Cloud services like Dropbox, or simply transfer the photos to your computer (i.e. into another folder or import them into photo management software such as the Photos app on macOS), and then delete them from your phone and these photos will still remain on your computer or Dropbox.