My iMac was bricked with endless loop as well this morning after a reboot (on latest beta) and I couldn't even access the restore features. It had to take it to Apple today and their machine can't even read it!
I found that macOS couldn't write to the hard drive and that would crash it every time. I too wasn't able to get access to the recovery mode because the Mac wouldn't recognize my bluetooth keyboard in time. I unplugged it for 15 seconds, it then started recognizing the keyboard some of the times. I was able to reset PRAM by using CMD+Option+P+R (hold it and keep holding while it reboots and you hear the startup sound twice). If you have a USB keyboard, use that instead. It's more reliable.
I don't know if this is your issue as well but what fixed it for me was splitting my Fusion Drive. The SSD was corrupted and failing, holding back its internal HDD sibling. Once split, the HDD worked fine to install macOS.
If your keyboard is recognized, try holding CMD+Option+R to boot into internet restore where your iMac will connect to Apple's servers and download very limited code to at least get you Disk Utility and from there, allow you to reinstall macOS.
Another alternative if you have another Mac would be to hold T on startup to go into Target mode. Connecting your working Mac to your broken iMac will give you access to your hard drive where you'll be able to try to save any unbacked files and then wipe your iMac's hard drive and reinstall macOS after that.
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OP wrote (in reply 10 above):
"But I stuck to my guns and my intuition told me that it was a corrupted FusionDrive, not a broken harddrive. I can’t believe I did it."
So... what are your conclusions?
Is the SSD portion of the fusion drive... dead, no longer working?
Is the HDD portion still good, still working?
Yes. The SSD was failing and since macOS uses it for core system files, at one point, the SSD failed to write a system file and crashed the system. When it automatically rebooted, macOS was no longer able to write and it went into an endless loop.
If you're "up and running" from the HDD, you might try erasing the SSD and then testing it using Disk Utility.
What results does DU give?
Can the SSD be "revived"? Or... no?
After splitting them, both the HDD and the SSD are writeable. The SSD still shows a S.M.A.R.T. warning. I won't use it for anything sensitive but I'm thinking of using it as my Photoshop and Final Cut Pro scratch disk. If it fails, the most I'll lose is some work since the last save and the app might crash.
Again, if you're booting and running right now from the HDD, please re-read the advice I gave in reply 5 above.
Someone else said you have USB3 on a late 2012 iMac.
Everymac.com confirms USB3 on the late 2012 iMacs.
If this is the case, again, you will be amazed at the performance boost you get by booting and running from an external USB3 SSD.
Get a modest sized Samsung t5 (even 256gb will be fine).
It will be WORTH THE MONEY SPENT
I'm going to consider doing what you suggested but right now, I'm just milking my 6 year old iMac until the last drop before buying my next iMac, likely an iMac Pro. I'd rather put all money towards that.
The iMac without the SSD is in fact noticeably slower, specially at waking from sleep but it does get the job done. I haven't installed Mojave yet, but I did notice it was way faster than Sierra, even running off an external spinning disk drive. Mojave apparently delivers on Apple's performance claim. If that works, I should be ok until the next update to the iMac Pro is released in a year or so.
