How can I regain an admin account on my iMac?
I was able to do this in Recovery mode, not single-user. In Recovery mode, you are not started up from your normal system volume, so files are not at their usual paths. 

I had to go into terminal and type 

rm /Volumes/"Macintosh HD"/var/db/.AppleSetupDone 


Note: that if your main volume is named something other than "Macintosh HD", you need to substitute that at the appropriate place; also, there must be a space after "rm" and cannot be one before ".AppleSetupDone".

Also, standard dscl commands will not work as expected in Recovery mode. Once done exit terminal and restart. Upon restart I was able to select a new user with admin privilege. 

What a beating !!