pabeley -
The full document is now visible on the Future Luton - Optimisation website. The additional pier would service the south-west side of the east apron, a new six-pack parallel to taxiway delta and the south stands, widened at both ends to create 6 stands. So it will be a long and busy walkway, presumably on two levels to segregate inbound/outbound passengers.
The new taxiway is a dualling of Delta, not Echo, between Alpha and the mid-point of the east apron, so impinges on neither car-park nor Harrods.
You are correct in your analysis that there is a conflict between the interests of the council (more passengers) and Abertis (cargo and exec flights). What Britain, the South-East and London needs is greater passenger capacity. Exec aircraft have far more options, even in the south-east.