The Planner is quite right about how various things line up in reality, as against a nice neat computer diagram. And trains can cross from the down loop on to the Bicester line at the junction.
There just isn't the space available on that site to have six through platforms. You might at a pinch get five, with two islands and a single face, but three islands is not going to happen. And the connections into the stabling sidings from the north end were removed a few years ago.
With the projected increases in container traffic to and from Southampton, I'm of the view that keeping freight through roads, to keep Freightliners going at full tilt well away from passengers of a nervous disposition, are a pretty good idea.
As I've said back up the thread, the draft Western Route Strategy highlights concerns about capacity between Oxford North and Didcot from 2019 anyway, with what is already expected to be running then, never mind Cowley services, with quadruple track and flyovers at Oxford North and Didcot expressly mentioned as potential solutions.