Thanks for the detailed answer.I have tested your issues on xwlpi and on my I9300 many many times now.
Rejecting calls using power button was not possible ever! Neither on SGS2 nor SGS3.
Ending calls using power button worked 100% of the times.
Never experienced the receive call issue. Sorry. I think your phone is faulty somehow.
Version v14.1 is mainly bug fixes. I have never messed with the basic phone functions or settings. Also never modded the phone.apk either.
Indeed I couldn't swear that Power rejected calls, although I did have the impression it was a way of rejecting incoming calls - I was wrong. I started instinctively using it when I got 3 or 4 times that situation of only seeing the wallpaper when the phone rang. I haven't been able to reproduce it further (for the good and for the bad).
As for disconnecting an ongoing call with Power but instead having the screen turned off and when coming back on having the green bar at the top, like when going to the launcher, it also hasn't occurred since.
I'm hoping this might have been some transient situation that the phone app might have become inconsistent somehow, and perhaps all these occurrences where in the same session before I rebooted (don't remember).
Thanks again for your support!