When you arrive on the ship, check with your room steward or at the Purser's Office regarding changes in dining. The maitre d' is usually available in a small room on the ship to accommodate special dining requests prior to the sailing. It is not usually well-publicized and the information is not widely available to avoid having everyone on the ship make changes to their dining arrangements.
As far as smoking vs. non-smoking sections, one side of the public rooms is smoking and the other side is non-smoking. Unless you have a table that is right on the divide between non-smoking and smoking, it should be tolerable for most people. If you can't get into the Princess Grill, again, speak to the maitre d' and request to be moved to a table further into the non-smoking section.