Once more thing, I would go to the settings page on the iCloud site (
https://www.icloud.com/#settings) and on all the devices listed, go to iCloud settings, manage storage, and tap "Disable and Delete" in the iCloud Photo Library section.
Do you have any more devices shown on
https://www.icloud.com/#settings than you actually use?
After doing that, if the rogue data persists, I would set up a new iCloud Photo Library on all your devices with a handful of photos, to see if this will reset the storage counter and/or clear the data that shouldn't be there.
Failing that, a call to Apple support it is. (Rather you than me!
)