Is there a key command to shift Apple Mail focus to top?
You should be able to press the Home key (Fn + ← left arrow key on a laptop) to go to the top.

Alternatively, if you have a multi touch trackpad, you can use BetterTouchTool to set a gesture for Home and End. I personally think this is the best way, and I set a three fingered swipe down to Home and a three fingered swipe up to End