Was this clean install? I tried to clean install due to issues with Sierra (wanted to revert back to old OS).

Activated erase the Macintosh HD, picked (like idiot I am) the highest erasure mode. It took 10 minutes and nothing moved... I thought I overdid, let's try again - forced shut down to begin anew. (Yes, I am an idiot...)

Opening Mac again using Cmd+R, Macintosh HD was now inaccessible. I tried to recover what I could, but it was gone. Somehow I managed to change it into Apple Disk Image I think - one of the four "disks" that appears when entering Cmd+R and going to Disk Utility. (Now I had two Apple Disk Images or whatever its exact name was)

I partitioned APPLE HDD and made new Macintosh HD, but that seemed to be empty.

Now I decided it was safest to just call a technician and had him install El Capitan back on and stop being an idiot.

Got my Mac back, it was like first time - appoint my admin user name etc. Those things Mac asks when you get it first time and make it yours.

I'd like to know: does this sound like clean install was done? If there was (doubt it, but just in case) malware or trojan installed, was it wiped out and system is now clean?
This was definitely a clean install, also known as an 'Erase and Install'. Any application or extension-based malware (or trojan) would be gone at this point; it was gone the moment you erased the drive. As for the theoretical firmware possibilities another user mentioned, those are highly uncommon and I wouldn't worry about it.