My answers in bold, below your sentences
You are working with your own compilation then, I assume?
No, I am working with the official stable ArduCopter Version.
Meaning gyroscope, accel, etc? What happens if you try to calibrate them?
I can't even calibrate. It is just showing nothing and not responding.
In top you should see apm, not ArduCopter.elf. Please run the following and post the results:
sudo systemctl status apm
As you can see, if I run top ArduCopter.elf is running.
Also, do check the PID doesn't change (it would inicate it's trying to relaunch the script continiously, indicating it's not getting launched correctly).
So the PID is changing constantly, what can I do about it.