Likewise I can't see alternating trains on the relief lines formed of 9-car Crossrail, then minimalist GW formation. GW currently regularly send out 2-car sets with standing loads, that attitude will not be allowed to continue. The milksop joint operation will be subsumed into a Crossrail operation.
Regularly? Where? The Greenfords?
You make it sound as though GWR couldn't give a monkey's - whereas the root of the problem is the lack of new dmus for Paddington services since, er, 1993. They do the best they can with what is to hand and if a two-car set is going anywhere other than Greenford in the peaks that tends to mean a set has failed somewhere, resulting in a coupled formation being split to provide some sort of cover for the failure.
There should be plenty of stock to go around once the 365s and 387s arrive - all of which should be staying with GWR once Crossrail starts running - so there aren't likely to be many if any lone four-car emus running around in a couple of years' time.