The bigger issue (wrt bandwidth usage) is the Photo Stream uploads, that can be switched off separate from the normal iCLoud backup. The iCloud backup is incremental, once you take the knock from the initial backup, it not that much on a daily basis.
My iCloud backups usually run between 0:00...
Nope sorry, but if the iPhone 6 silicone case is anything like the iPhone 5c silicone case, I think the leather is nicer since it doesn't attract that much fluff in your pockets.