There is still one question I haven't seen you address yet, JMCC.

Is it more or less expensive to build and maintain a system that supports single-decker rolling stock?

As presented, it would appear that single-decker is cheaper.

As the video makes clear, there is no drama attached to dual-mode co-existence, given that a good many of today's passengers change trains without giving it a second thought. So it may reasonably be concluded - unless you have evidence to the contrary - that the NorthWest extension will be cheaper both to build and to operate.

By the way, the whole tone of the Dempster report smacked of beat-up.

"Critics say this will lead to a break-up of the integrated double-deck public system designed by John Bradfield"

Bradfield had nothing to do with the introduction of double-decker trains, so could hardly be described - as the piece represented - as "turning in his grave". Unnecessary hyperbole.

He could be blamed, albeit indirectly, for building a system that encouraged the development and usage of rolling stock that was completely incompatible with his design. Without double-decker trains, there would be lower stress on the track. With lower stress on the track, there would be less need for continuous, and expensive, maintenance and/or replacement.