Oh wow, that corrupted T2 firmware seems horrible. I have a brand new 2020 13” and I will make sure to turn off secure boot. What I’m wanting to do with this old machine is to run Mohave on it via an external drive. I wonder if I could do this if I wipe the machine and start over. There isn’t any data on it that I care about. All my data is on the external drive. I just need this machine for a handful of old files I need to edit that will not run on Catalina. @bowline does this change anything?
If Startup Security Utility has Secure Boot set with Disallow Booting From External Media then you shouldn’t be able to run Mojave or any OS from an external drive.
If you have a bootable external drive somewhere (maybe a backup clone) then try it. But if the machine is locked down and you cannot access Startup Security Utility you may need to contact Apple Support.
Kinda’ important to state at the very beginning! 
Since you have El Capitan, when starting up using command-R, this is the menu you should see:
Which Mac do you have? The following is from Apple support:
Maybe, when you did see the Startup Security Utility on April 13th, you had started your Mac in recovery mode through one of the alternate methods. I’m not suggesting you do this now. Maybe someone here could come in about that.
Finally, since you initially tried to boot up your Mac from a SuperDuper clone (your initial post), you may want to contact SuperDuper support.
https://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html
@bowline do you recommend no security, or medium security?
On my new machine (2020 13" MBP running Catalina) I was able to turn it off. But now it’s saying the OS on my SuperDuper backup needs to be reinstalled. All I really need is a machine that runs Mojave so I can still open these files while I wait for the developers to update.
Thanks so much!