HD failed and need reinstall of El Capitan - help!

I am somewhat out of my element with all this, but here goes:


Today my iMac was just hanging for eons and I kept trying to reboot.  Finally I did Command-R and tried First Aid.  First it said the Mac HD couldn't be verified completely and the repair failed.  Then I look at the info and the volume said that it it had 0 available space and all its 1T capacity was used (which is impossible - I only have about 450 GB on there).  I took screen shots in case any of that info is helpful.


After trying this a few times, I exited and it wouldn't complete restarting, but would go with the apple icon about halfway and shut down again.  Thankfully I make a weekly bootable SuperDuper clone and could boot from that, and there was a warning on the screen that said "OS X can't repair the disk "Mac HD.  You can still open or copy files on the disk but can't save changes to files on the disk.  Back up the disk and reformat it as soon as you can."


Obviously I can't do that since it won't let me log into Mac HD as I presume it somehow has been completely damaged or failed.


Anyway I have my computer up via last Sunday's SuperDuper bootable clone, however it seems I need to try and "Erase and Reinstall OS X" on the failed internal Mac HD.  Yes?  If I try that it appears that my choices from Disk Utility would then be to "Reinstall OS X"....  my fear is that this will then download the LATEST OS X - which is High Sierra.  I am running El Capitan because I have apps that still need that and do not want High Sierra. 


I tried to see if I could get the El Capitan Installer, but if I try thru the App Store it either says "You can't install it on this computer" (don't know why since I'm already running it (via the clone I'm now on") or it says to go to past purchases, which I see it from Sept 5, 2016 and if I click the download button it says that if I want to download a copy of the installer to continue (which I do.)   Then then download button goes grey and NOTHING HAPPENS.  There is no downloading, there is nothing.


I am pretty much at my wits end.  I tried to look at my Time Capsule to see if I could find the installer from back on Sept 5 2016 but the Time Machine doesn't let me access anything while logged into the bootable clone.  (No idea what the point of a TC and TM is if its not bootable and you can't access it except from a failed HD, but I digress.)


So my questions are -

can I "Erase and Reinstall" El Capitan from Disk Utility or does it force the latest OS?

why can't I download a copy of the installer that it sees I bought from the App Store?

what if the HD is just failed and Erase and Reinstall does nothing?

I would just go buy a new Mac but they come with High Sierra pre-installed and I'd be in the same situation.


I appreciate any help someone can give me.


Many thanks!

iMac, OS X El Capitan (10.11.6)
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If you erase and reinstall it will just be system that reinstalls and you'll need to migrate from your backup disk.

However that step would be a good test of the disk because it will be a clean Mac OS. Then if that works you could migrate back:

How to move your content to a new Mac - Apple Support

If you replace the drive, and clone it will be an identical copy of your present system and will have everything you need, including all of your data and programs.

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