Q: Free space of iCloud Photo Library

It either will settle by itself after a week or so, or you may have to talk to Apple support, they may be able to cancel reservation that is created by library for something that was uploading and then you may have deleted.

Try a simple Reset: hold down the Home and Power buttons until you see the Apple logo (10-20 seconds).

If this doesn't help, you might need to Restore your iPhone.

https://support.apple.com/en-us/HT204184

Do you have any Shared Photo Albums? How about Photos or videos in Emails or Messages you have received?

Cheers,

GB

Once you delete the photos and videos, double-click the Home key and close the Photos app by swiping up. Then check your storage again. Also, in the Recently Deleted folder, have you done the following to make sure you don't have residual photos that were left back in iOS 7 due to an issue with the aging off of deleted photos:

1.     Go to Settings>General>Date & Time, turn off Set Automatically, and set the date to 09/01/2014

2.     Go back to Photos and open the Recently Deleted folder to see if you have residual photos and manually delete them

3.     Go back to Settings>General>Date & Time and turn Set Automatically back on

Cheers,

GB

Please see this: Re: New update iOS 9.3.2

Still advocating against forced restarts, perhaps the wave of the future.