Whilst I don't know all the definitive answers - a friend was downgraded to Economy from CE recently on a flight to AMS and then didn't have access to the lounge!
He could have taken the next flight in J but chose to fly earlier on his original flight!
I do not think he pursued IDB, but did speak to CR about it later and was given 2,500 avios IIRC.
So no lounge access if downgraded!