Does the cab end fully fit in the platform at coryton then? I thought it had a short platform for even a 3 car set.
The new (but currently covered up) stop sign is partly down the ramp. A 3 car 756 stopped there will have the rear (in the arrival direction) cab fully on the platform.
The plans looked as if they were for where the rear cab would end up for a 4 car train. They were for a short separate platform, not an extension of the existing one.
One other possibility is that they plan to run 398s - a four-car 756 is about the same size as two 398s coupled together.
Perhaps but I don't think I've ever seen a suggestion from TfW that they would.