If you want snapdragon 820 for gaming then just wait for google pixel (nexus) phones . It might give you snapdragon 821 for your gaming or if you want snapdragon 820 go for oneplus 3 affordable gaming plus with 6 gb ram .
There are some issue 32 bit devices . Therefore it might not work yet but in future may work. In review section of app facebook mention this issue. It will help.
Yes you can use,If your micro usb and type c are genuine or compatible with your phone . Then there is no issue. But if your using not gunuine products it might be some problems . During attachment (not reading properly , etc.)
It might aux cable problem due to wire cuts or just go to service centre for headphone jack checkup. The headphone jack can be broken because put enough pressure on headphone jack , or suddenly in fast move you detached the headphone jack.
Yu devices are in beta testing . For their marshmallow software they called them AOS ( Android on steroids) for old devices . Therefore marshmallow update is not available.
Vulkan api just bridge between hardware and software. As compare to other api. Vulkan api reduced distance between hard ware and software , therefore you get more power and efficient . Performance.
Just go service and get checked the phone, same incident happened with me. After i find that display internal parts get broken. I suggest you to service centre for check up.
Actually vulkan api used in every heavy games . Just like asphalt 8 or nova 3 like. You say it can used in every game. It just like directx of windows os.