You might be experiencing a problem with incompatible headphones. There are several incompatible headphone types that use different control signaling techniques. Here's the first google result that I turned up on this topic, http://forums.windowscentral.com/windows-phone-8-how-guides/...
I very much doubt that every headset I tried, including several marked on the package as Android compatible, were incompatible with every phone I've had. And It wouldn't explain the delays for Bluetooth headsets or the lockscreen button.
