This happens when you have a newer ver installed than is available. Since you've installed a ver from bpo, and now commented out backports in sources.list, you can't reinstall, and you can't get any needed dependencies. It looks like you have marked pkgs to be installed form backports, but before that could happen, you commented it out and apt updated the available pkgs....
Reinstallation of nvidia-kernel-dkms is not possible, it cannot be downloaded.
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There's a 'backing out in case of failure' on the nvidia wiki. It might help you get it all removed, then start over. I would try the stable ver first, spend a little time on it, and if it absolutely will not work, then I'd remove it, and try the backport version. When using a backported video driver, I'd probably use the newest backported kernel too. That's one possible thing to try now, since you have the headers, and the dkms, why not reenable backports repo, install the 4.18 backported kernel and see?
Hang in there, and when you get it working, let people know, thanks.