I'm not quite sure what exactly is creating your Documents & Photos folders, but it is not the "Camera roll bonus". This bonus was added to your account when the automatic upload was active once and it is by design that this wont disappear after you deactivate it.
This bonus storage will disappear by January 31st 2016 if you do not object.
Also, this ...
Unfortunately there is no way to automatically re-upload all images in best quality. You could either manually upload the old photos through the OneDrive app on your phone or go the USB->PC->OneDrive way. Been there...
I don't know if I really understand but:
In the Pictures Hub just below "Camera Roll, Albums, etc..." you can se a "upload(s) waiting" message.
Tap it and all pictures waiting for upload will show.
Long press an hit cancel. I think you can't do multiple actions here. Need to be one by one.
As you already be suspecting, that is not possible. The operating system is trying to protect people from themselves and do not allow people who may not have a good data plan chew through it too quickly.
Windows Phone / Windows 10 Mobile requires that you're signed in with a Microsoft Account in order to anything really useful (syncing/email accounts/etc). The fact that the last owner has auto OneDrive upload enabled means they are still signed in on that device.
You should remove that person's account from the phone immediately because they can track you ...
I dunno which app you used, but there are two new Fb oficial apps by Facebook Inc. available on store. In total , there are 4 Facebook apps in store.
Facebook by Microsoft Corp.
Facebook beta by Microsoft Corp.
Facebook by Facebook Inc. &
Facebook (Beta) by Facebook Inc.
Give 'em a try!