6tph on North TPE is due to happen at the December 2016 timetable change. Electric services between Manchester and York are set to happen at the December 2018 timetable change. If Hull is electrified it'll likely happen after April 2019 and the HLOS is for Control Period 5 which means the changes will happen between April 2014 and April 2019, so the HLOS is still correct even if post-Hull electrification Hull gets 2tph to Manchester. Maybe the reasoning for the one remaining diesel service on North TPE going to Hull and not Middlesbrough or Scarborough was that Hull was the mostly likely to be electrified all along?
I think more to the point is that Hull's urban area has a population of over 570k (compared to Middlesbrough at 138k, or Scarborough at 50k) and that while it seems to be politically unacceptable for Middlesbrough or Scarborough not to have a direct services to Leeds, it seems to be equally unacceptable for Hull not to have a service to Manchester (interestingly there is little call for it to go to the Airport...).
This is me indulging in some speculation, however if when considering the service plan you assume that ultimately Hull would be electrified, there is then a good case for extending the Selby to Hull too. You've then got eastern termini of 2tph Newcastle, 2tph York and 2tph Hull.
I have to say I'm not sure you can be as certain as to say "If Hull is electrified it'll likely happen after April 2019". If Hull Trains proposal is approved soon (ie in the Budget), then the delivery plan sees electric services starting at around the turn of 2016/2017. NR LNE are also considering bringing the committed east of Leeds work forward, perhaps to as early as to allow electric services Leeds to Selby, before to York!
With only rarely using on the Leeds-Selby-Hull line I've not taken as much interest in why Selby was picked as the terminus. Has it been confirmed that Selby-Leeds will get an additional service or could we see the Calder Vale diesel services no longer continuing to Selby once Stalybridge-Selby gets electrified?
For the moment Selby was chosen because Network Rail London North Eastern have prioritised providing diversionary routes for the East Coast franchise. Specifically allowing electric trains to reach Leeds via Hambleton South, Hambleton East, Micklefield and Crossgates. Once you add as far as the Hambleton Junctions, Selby isn't really that far and gives time benefits for the stoppers in this busy section.
As far as I have seen there is nothing to say that the TP terminator for Selby will replace anything else, and I would have thought it'd be a bit odd for it to turn into an all stops at Leeds, but we will see.