You may have to resort to uninstalling, running the cleanup tool, rebooting and reinstalling from your online account in this case because it would appear that things didn't go smoothly.
Exactly how my issue started - applying updates - firstly casued the "Realtime scanning turning off" issue occured - which forced me no other option except to re-install.
Then during install - I had the mfedisk.sys missing error.
Every year and every few months - seems more and more - that we need to protect ourselves from McAfee -and not viruses !!
Are you using any registry cleanup tools at all? Or using System Restore a lot? This is typical of what can happen. When it becomes corrupted like that the only solution is what I previously suiggested.
I've never experienced that myself and I have McAfee installed on 4 systems at the moment.
In general when upgrading it is best to uninstall, cleanup, reboot and reinstall.