I think part of the "problem" is that unlike most phones, this one has an external camera module. Instead of heating up the back of the phone, only the module gets hot and it will feel hot because it's made of metal. The material used for the module probably also plays a part on the higher temperatures.
I haven't seen anyone complaining about the camera not working, so I guess it's not a big problem.
Regarding the motherboard issue, unless your phone is restarting, you are fine. If it's restarting, send it to ASUS and they'll replace it for free.
My suggestion: use and enjoy your new phone. The "anxiety" of something breaking down will spoil your experience. If a problem appears later on, deal with it then. Right now, you should record as many videos and take as many pictures you want. I mean, that's why you bought the phone, right?