If I answer the call on my Watch then slide up the handoff icon on my phone it immediately transfers the call to my bluetooth. Not an elegant solution but better than nothing.
I'm with the OP on this one, if I'm going to check my watch for caller id my instinct is going to be to click the green circle answer button on my watch face not reach around my bluetooth headphone cord trying to find the center button to answer the call. I wish there was a button to select where the call is answered.
I haven't tried pairing my headset with my phone (I use spotify not itunes music) but I assume if you're headphones are paired with the watch answering a call would route the volume to the headset then.