If you look at the guides that have been posted you will see how to take apart the phone step by step and reach to where the audio jack is found. If you have other issues please refer to Xiaomi Mi4 Troubleshooting.
To remove the battery, this will take some time and some technical tools to do this properly. There are a number small screws that will need to be loosened along with disconnecting very delicate wiring that is connected to the logic board. If you have other issues please refer to Xiaomi Mi4 Troubleshooting.
Yes this is bad. It can mean multiple things that are causing this problem. These problems can range from the overcharge of the phone, dust built up inside that is affecting the ventilation along with software problems that are having the processor on the logic board to constantly being on. If you have other issues please refer to Xiaomi Mi4 Troubleshooting.
You can install a camera yourself however it will require specific tools so as to not compromise the device. Spudgers to remove the back and a specialty screw driver (Phillip's Head Screw Driver 3.0) to remove the tiny screws from the inner frame. After removing the inner frame, use a tweezer to detach the camera connector and replace it with the new one that you have ordered. If you have other issues please refer to Xiaomi Mi4 Troubleshooting.