It is probably not a firing problem if the mis did not move after swapping coils. I would check into the injector because it could still be bad even though the resistance checks ok. You need to listen to it and compare it to the other injectors. It would also be nice to have a noid light to plug into the injector connector to make sure it is being cmmanded to fire. I would check those things before I went on to a compression test and or leak down test.

 

If it were a head gasket problem with a miss, you would either be overheating because of the internal bubbles it creates, or like you said, the coolant would be disappearing.