We have swapped seats in different cabins with family members many times. Correct me if I am wrong, but what they don't want you to do is have two people swapping cabins throughout the flight such as one person eats the meal while another comes and sits for drink service. I have had no problem sending a drink back to a family member sitting in coach. Sometimes the FA will have me go back and deliver it.
Most importantly you both shouldn't worry about it for SEA - SFO since your trip to Greece in far more important than being separated during this short connection.