Deleting photos off of my iPhone but leave photos in the cloud

No, as far as I know, there's no way to do that using iCloud, they're always kept in sync. You can either:

• Ask Apple to change this behaviour: http://www.apple.com/feedback/
• Use a different cloud service, e.g. OneDrive, Dropbox or Google Drive, they all provide a way to backup your photos and sync them for offline viewing when needed.

Photos in the Camera Roll folder are the photos stored on your phone. Deleting them won't affect the photos uploaded on iCloud. However, you may want to check if your latest photos in the Camera Roll have already been uploaded on iCloud by checking the My Photo Stream folder. The My Photo Stream folder is the one synced throughout your devices.