OS X El Capitan Installer.app has crippled my Mac Mini Mac Mini (Mid-2011), 16GB SSD, 16GB RAM, El Capitan (v10.11.6)

The Mac ran fine until I upgraded to Sierra. The latest Mac OS conflicted with most of the specialized apps I use every day, So I immediately returned to El Capitan.

Activity monitor shows that two "Installer/Installer.app" items are always running and are they major users of all of my available memory. Consequently, all my others apps become unresponsive.  I run Disk Utility, and the Installer.app show up there as .dmg files; but I cannot erase, delete, or unmount them. None of my 3rd party app can remove the installers.

My Mac Mini, which should be running super-fast (as it used to), now runs dreadfully and disgustingly slow. I a truly upset with Apple for this kind of garbage. To get their help: $$$.
Double click the process in activity monitor and look for the Open Files tab.  This should give you an idea where the process is coming from 

Try to force-quit the process in activity monitor.

Then try to delete the dmg files (make sure they are unmounted, right-click > unmount on the dmg).