Sometimes mission statements go thru several re-writes over the first few years of being utilized, however it should become the backbone of your business. I would guess that your advisor is demanding some due diligence on your part so that you begin thinking like a businessman instead of just going out and starting to cut grass, and hoping the business part will follow later.
Kudo's to your lawyer for trying to nudge you in the right direction. Not many lawyers are that smart. Sounds like this chap actually wants to help you instead of just doing the work and sending you a fat bill.