Erase is greyed out so can't erase HD in Disk Utility

Have you followed exactly the instructions in the Apple Support Article your post refers to? If you haven't and you're trying to use Disk Utility whilst logged in with your account on your iMac then what you describe is exactly what you'll see. In other words you can't erase the HD whilst the system is still active on that drive. It's like trying to perform open heart surgery on yourself whilst you're still awake and walking about.

Restart with the Option, Command, Shift, and R keys held down.

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