I think some of the problems associated with the existing platforms and general movement around the station is more linked to the underground station than it is with the mainline station.
The loigical, and shortest, route to the Picadilly and Victoria lines (from platforms 0-7, 8 has direct access to 9 so no need to walk to out the front) is to exit the main station and down the steps, turn right and down the escalators. This, of course, is now blocked forcing users to walk to the new northern hall. This is a very long walk so users are trying to shorten it by using the platform escalators and entering the northern hall directly. This saves a good few hundred meters.
In fact, if you arrive at the rear of a train in to platform 0, the sign posted walk to the Victoria line takes ages. I would guess about 2/3 mile.
There are two options which are shorter (and allowed - although sign posted).
Option 1 - leave by the eastern exit and enter at the old Thameslink station. The Victoria line is very close to this entrance.
Option 2 - walk out the front of the station and cross the road to the St Pancras entrance (or go down the stairs directly outside). Then pass through the Cirlcle line gates and follow the signs to the Victoria/Picadilly/Northern lines. This is actually the old hall (the one that is blocked to entry from the passage ways).
Trying to force users on very long walks is never going to work with regular commuters. Airport style one-way systems just don't work in railway stations.