How to revert unsupported macOS update (Monterey to Sequoia) on late 2012 iMac?

Ask your " cousin " to fix this issue they have created

From your above description, this computer is running a " Hacked Version " of macOS

Normally, I do not offer support for running a Hacked Versions of macOS

But being as your " cousin " is involved and you have taken the " High Road " and what to put " Things Right " again

What to do before you sell, give away, trade in, or recycle your Mac

Or, If you what to the 100% Sure all the Hacked Version is 100% Removed

Scorched Earth Method 

This method will WIPE ALL Data and there is No DATA Recovery - Period.

To perform this action will require booting from a Bootable Installer. 

The Bootable Installer can Only be performed on an Apple Computer 

This will have to be performed from a Qualifying Computer to run the version of macOS to be made on the Bootable Installer.

Example : Bootable Installer of Catalina would have to be done on a computer that Qualifies to run Catalina .

Notation: If the computer being used to perform this action is Too New or Too Old to qualify to run the version of macOS - this computer can’t be used.

Alternative is, to gain access to a Qualifying Apple Computer from a family member, friend or associate.

Shutdown computer and disconnect all external drive Except the newly created Bootable Installer.

Restart and immediately hold the OPTION key until the Startup Manager appears and choose the USB Drive. 

It will present options >> Disk Utilities >> View >> View ALL attached Drives. 

Choose the Upper Most Drive ( not the volumes indented and list below ).

The drive normally is called Apple Media or Apple SSD - that is the drive to Erase and format as APFS with the GUID Partition Map.

Once that is done >> backup out of Disk Utilities and choose install macOS. 

Follow the prompts and it may automatically reboot several time. 

Upon a final reboot - Setup Assist will present with the Un-Hacked version of macOS.

First, make sure to create a bootable macOS 10.15 Catalina USB installer now while you still can by using the instructions in the following Apple article:

Create a bootable installer for macOS - Apple Support

You will need to perform a PRAM Reset (hold it for at least three chimes) in order to clear out the customizations needed for the hacked OS installation to boot.

Once booted to the installer, you will need to launch Disk Utility to erase the whole physical internal drive. Within Disk Utility you need to click "View" and select "Show All Devices" before the physical drives appears on the left pane of Disk Utility. You need to erase the physical drive as GUID partition and either MacOS Extended (Journaled) or APFS (top option).

If your iMac uses a Fusion Drive, then I would suggest erasing both the internal SSD & Hard Drive to destroy the Fusion Drive setup, then use the instructions in the following Apple article to rebuild the Fusion Drive:

How to fix a split Fusion Drive - Apple Support

If it has a Fusion Drive, then once recreated only use Disk Utility to erase the Fusion Drive item although this is not necessary since the creation of the Fusion Drive will also format it with a file system (Catalina should be able to use either MacOS Extended or APFS for the install process).

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