...2 is the split of the 3 portions at Edinburgh just a simple matter of taking away the 3 portions 1 by 1, maybe adding a seated coach ?
As well as other responses, see links below...
3 is there somewhere I can look to find out the way the stock is formed prior to departure from Euston ?
If by this you're meaning the shunting of the stock, that is all done at Wembley InterCity Depot and the 16-coach train comes in from Wembley hauled by the ECS loco as one train. I don't think Wembley is all that visible to the public.
Unless there's a need to switch a defective coach etc. I would also have thought they just keep the 16-coaches together having been brought in to the depot from Euston as a full rake after the previous night's working?
PS I think it might be a good idea if CS did their own short videos for Youtube with commentary , maybe showing any shunting at Edinburgh plus l
some lovely views going over Rannoch Moor (Ive done it by DMU )
I could possibly see CS using footage of the Highland scenery to promote their service (indeed there's already plenty of stills on the website) - however I'm not too sure the shunting around of coaches in Waverley at the dead of night with lots of noisy diesels to-ing and fro-ing is really the image they want to project/sell! ...that's more for the enthusiasts!
Fortunately there's already a few YouTube videos of the Edinburgh shunting, some examples being as follows (they're from Scotrail/DB days, but the order of events is still pretty much the same):
https://youtu.be/tBpMMw-ecDk Northbound Highlander splitting at Waverley
https://youtu.be/5rMxkPUu2dY Southbound Highlander forming up at Waverley
...and then a good one for your potential FW trip - "Up all night on the Caledonian Sleeper" - this includes footage of the Edinburgh shunting (from 1:11), plus some of the Highland scenery:
https://youtu.be/UYj06jvgM8c