It depends on the lighting
Try and go outside, on a sunny and bright day, point at the clouds or something, and zoom
Then cover the center camera, it should be using the 2x lens (can confirm with my SM-N960F)
Do you have a magnet close by the phone? It interferes with the spen tracking
can you specify where exactly is the "small part of the screen"? dial *#0*# and test the s-pen?
You won't pass safetynet if you're using xposed (I don't think there's any way around it but uninstalling xposed)
- The phone likely assumes your output is a pair of headphones, and reduces volume as a safety feature when listening to media for long periods of time at max volume (that's what I'm assuming), since it damages your ears if you're outputting to a ...