Are you using windows or android ? both speed control should work. on Android, there are certain phone does not support, our next release should handle this problem.
on windows, you need to check if your PC has personal router enabled. when this happen, it usually means it can only cut off not speed control. we have plan to release a fix for this , but need wait after android.
no, the phone guy would not find out , unlikely , unless they step in and you are running at same time.
however, hey, you suppose to use necut on your own family where you are as a dad or mom :).
If the internet are too slow while you trying to do something impotent, you could talk to you family and make sure the speed you use netcut to control is enough for the rest family.