iPhone 6 Plus overheating bad?

Was the phone purchased from a different owner? If not, I would definitely start a support call with Apple. If you still have a warranty on it, you will want to use it. There can be so many things wrong with the device which are hardware related. If your phone is getting extremely hot and has a bad smell to it, that can be very dangerous and sounds like the battery is what you are smelling. As for the cameras, that can be a short somewhere in the device or just bad cameras. The overheating is what can be dangerous and can potentially end up hurting you. If you have a backup phone, i'd switch to that and try to not use your iPhone until you can get things situated with Apple. If you have a repair shop in the area, give them a call, but most likely it's something they won't know how to fix. So just call Apple and see what they can do. If they can't do anything, try giving your local phone repair shop a call. If they can't do anything either, I would just get another phone and recycle that guy.

Please note: I usually won't tell anyone to scrap their device, but having these issues can be very harmful if the device is repaired incorrectly and or still used constantly.