I think the slow lines to the west could be made to work, but I am not sure that it would be better. Thirsk rebuild, fettling existing southbound slow over nearly 30 miles, you've still got Northallerton to sort out, and the difficulties of moving services from Fast to Slow over the route, seems like a lot of expense for very little benefit, as I see it only benefit would be southbound freights being able to use the York avoiding line without crossing the lines outside York. If this move is seen to be a problem then a flyover north of York to address it, much less work than the pair by speed option. I would rather alter Northallerton so that it is 4 track through the station, with outer platforms on the 'slows' so that calling services can be kept out of the way of through services, and the ability to switch from slow to fast just to the south of the station. I think there is space if you demolished the northbound platform, but the cost may mean its not value for money.