I think that ATC should prompt 7700 and that it should be included on the check list for the pilot, it is a very efficient way of letting everyone know what is going on.
In the UK due to the way the airspace is structured with multiple sectors sitting on top of each other, lots of the those sectors will have height filters on. A 7700 will break through those filters and alert the underlying sectors before the initial sector can tell them themselves, this means any conflicting traffic can be moved out of the way in good time.
As for a turn of the airway, i don't think it is a good idea. If we know what the emergency aircraft is going to do ie fly straight ahead, then we can help more by getting the right aircraft out of the way instead of trying to predict what is going to happen.