Thank you Mario, for the good news. Regarding your points:
1. You mean after the phone call started the screen stays on? I am afraid there's not much we can do, since the BT button press will hang up the call at this point.
2. It takes a while until Google Now lets the audiofocus go. I don't think it is much shorter on other versions of Android. Sometimes it helps if you press the button more than one time.
3. Please test the same thing using the handheld microphone. I would be surprised if it worked differently with the headset. Maybe your available actions are limited if you come from a patter lockscreen? But they should tell you...
Thanks for the report, it's good to have somebody who can test for us on Lollipop.