Welcome 

New or used set?

can you elaborate abit how it happen or is just randomly happen?are you on stock rom or custom rom?

have you tried factory reset or maybe try rebooting your phone to see if the problem still persists...

seems like u removed some HTC Sense weather app or something.

that normal bro , it was real sensor , when your face near it will turn black because scare your face touch on the screen like " mute " " hold " "close " or some button on screen . that why when on call , sometime near will turn blank , u can try make a call then put your hand near the screen it will turn black ^^

that is the new Sense, you can always upgrade to AOSP ROMs, which removed all the Sense stuff

there is guideline to unrooted back the phone and stock rom, no issue on claim warranty.

if you're new beginner, maybe just stay at unrooted or stock rom.
I rooted because I need to modify system table partition and load custom rom.

anything you need to do now but rooted required?

Dunno bout Starhub set bro.
their got released their own RUU or not?
like when booting got Starhub bootanimation?

animation that running while on boot. that one we call bootanimation.