1. and 2.
if you try the shell command for input keyevent .. and check the option for the output to go a local variable
you will see that the shell output is 'Killed'
this is a message from the shell and says that while input keyevent .. was running it was killed or crashed (eg. due to some permission missing)
it is indeed possible that the volume was changed but this was just one thing that should be done, the other thing (bring the slider) did not execute because ... 'Killed'
about 3. read this
I am trying to simulate touches on my Xiaomi Mi Max 2. I tried monkeyrunner but failed, so I am trying adb. However, each command is returned by Killed response and no touch/key is pressed. What do...
stackoverflow.com
somebody said that he solved this issue with this strange permission
I haven't found any such permission !
but for completeness I must mention this
about 4.
there is scant documentation on sendevent, it's a low level operation
again I mentioned it for completeness
I doubt that it will fix the problem (it does the same thing that input does)
finally:
for me it's something peculiar to android because it seems that audio volume management by simulating the keys can be done only by an app in the foreground
that's why the app I told you exists
because there is a need by many people (like you and me) to bring the volume slider without keys (this app displays nothing but comes in foreground, it's a trick)
it would be interesting if someone with a rooted phone does this test and post the result