Given CrossCountry have committed to reinstating their hourly service to Stansted Airport and their full 2tph between Reading and Birmingham, and Birmingham and Sheffield, plus various swaps to fit in the extra service to Cardiff Central as a Voyager, it seems rather unlikely that there's the fleet for the lost Glasgow Central / Penzance / Southampton Central / Exeter St Davids extensions to return. Or rather the fleet is there but it has all been committed to the double formations of Voyager services. Four of those diagrams are there to replace the two HST diagrams.
If there's any flex in the fleet when the Voyager works are complete, it's very unlikely to be Glasgow that gets priority anyway. A bung of cash from Transport Scotland would probably change the situation.
50 mins? Really? The current LNER service takes 64 mins without a Carstairs stop. Where could you save 14 mins?
I think you've misunderstood. The current timing without any allowances from Motherwell to Haymarket is 38 minutes. Add 10-15 minutes to stop at Haymarket and then wait for a free platform at Edinburgh and you're already at a 48-53 minute timing today.
This was the norm for CrossCountry services even prior to the Carstairs works.