It is holding tightly as per my experience...
I don't really have good advice but I have used a very thin prising tool near the edges which I could open the case with.
...do not use sharp tools as it may easily cause physical injury to phone's body...
Anyway, if you could dismount the base, ensure to open it gently as fingerprint sensor's connector can be easily broken if it is still connected or not fully released.
I don't think any options exist to shut down the system properly without a screen.
By pressing power button, it makes a hard reset, so you are right, it is not t