There have been a few incidences of the cover being less difficult than
it appears, or if the pressure (force) is too great in trying, it jambs further.
This repair guide and comments tells about how or how it should be
removed... maybe a few serving suggestions, but similar recipe.
•Mac Mini Late 2012 repair guide - iFixit:
https://www.ifixit.com/Device/Mac_Mini_Late_2012
Most sections involving access for any all processes inside, start with
the removal of that metal cover. Perhaps you've tried too hard?
{I've yet to decide if my similar MINI, will be around long enough for me
to add RAM or not. Mine's a i7 'QC' server, sealed in retail factory carton.}
Hopefully you can talk nice to the Mini and it'll open up for you.
Good luck & happy computing! 🙂