Good afternoon,
To me this sounds like you know what you are doing and just need some help. That always makes our lives a lot easier :)
What i'm going to guess is that it's just super tightly sealed. What my recommendation is is to try and pry the side off to see if it will budge (of course, remove all visible screws first). The reason i mention this is that this phone would likely use clips to hold the back panel on, otherwise if adhesive was used it would never come apart. Failing that, the good old heating the back panel might work. Otherwise, heat the front screen, ***''++BEING CAREFUL++''*** not to poke at the display otherwise it's game over.
Good luck :)