To be fair the trains I've taken this week have been a bit quieter. I think you're way off base with the idea of halving the Edinburgh service being okay though. I've not been on a train that was less than 80% full south of Newcastle, and some of them were deeply off-peak
Really? I commute Durham-Newcastle and with flexible working patterns find myself travelling at a range of times, and that is not my experience of East Coast's services. I'd say it's only during disruptions that I can't find two empty seats next to each other, admittedly usually by wandering down to coach B. Things were busier last week as there was a reduced service (2tph Darlington-Newcastle) due to engineering works.
With regards to the wider timetabling: there's currently 13tph. Let's say that they can run 6 most of the time.
I'd run
* 1tph limited stops Edinburgh-Newcastle-York-Kings Cross
* 1tph Leeds - Stevenage (though see below)
* 1tph Edinburgh/Newcastle - Stevenage, all stops
* 1tph Other Long Distance (Hull Trains, Grand Central etc) - Stevenage
* 3tph Stevenage-Finnsbury Park-Kings Cross express shuttle
* 1tph Peterborough-Kings Cross all stops
* 1tph Cambridge-Thameslink all stops
This presumes that:
* There's capacity in the Stevenage area of course to put these trains somewhere! If there isn't, perhaps 1tph of the long distance services could terminate at Peterborough instead
* East Midlands Trains can enhance their Leeds-St Pancras service to at least 1 every 2 hours. If this can't be done, then the services from Leeds would have to run to Kings Cross, and potentially the Stevenage-Kings Cross express cut back to 2tph. Furthermore, if the capacity could be found for 7tph at Kings Cross, then I'd make extending the Leeds services the priority.