While inactivate updates are indeed kept in the Local SoftwareUpdate plist, scheduling data is kept in
~/Library/Preferences/com.apple.scheduler.plist
alternatively it can be in
~/Library/Preferences/ByHost/com.apple.scheduler.plist.UUID
Settings such as Automatic Download are typically found in
~/Library/Preferences/ByHost/com.apple.SoftwareUpdate.plist.UUID
I would isolate the scheduler plist since it isn't following what you set.
In addition, since it is launching at login, I would check your
~/Library/LaunchAgents and /Library/LaunchAgents in the event that peruser launchd is calling a launch agent which is calling for a Software Update check.