Just out of interest, why would you want to fix something that isn’t (in my view) broken? The three Highland portions have the furthest distance and the longest journey times, so it makes sense combining them in one train in the 21.15 path out of Euston, and then similarly the Lowland portions in the 23.53 path. These are the only paths available to CS as far as I can see. If you tried to combine Lowland and Highland portions in the one train ordinarily, you would either have Lowland passengers arriving ridiculously early, or Highland passengers arriving ridiculously late- I can’t see an 11.30 arrival in Inverness or a 12 noon arrival in Fort William being particularly popular.
You wouldn’t be able to combine Inverness, Edinburgh and Aberdeen portions as one train, as (although there is lesser demand for Aberdeen) both Inverness and Edinburgh ordinarily need a full six sleeping cars, and 16 coaches is the maximum that the platforms at Euston will accommodate. You could perhaps have Edinburgh and Aberdeen passengers sharing one portion of the train, but this would mean either a ridiculously early arrival in Edinburgh or the Aberdeen train waiting until a sociable time for the Edinburgh passengers to disembark- meaning an unattractively late final arrival in Aberdeen. In any case, I doubt anyone would be happy with a sleeper train taking up vital platform space in Waverley for hours on end at the start of the morning peak.
Also, combining the Fort William and Glasgow portions in one train is, in my view, a waste of a path (and where would you split them, and how do you ensure sociable arrival times at both destinations?) as even at the height of summer, the Fort William portion has only four sleeping cars (the train can only be six coaches long, including the seated coach and lounge, due to platform and passing loop lengths on the West Highland Line), while the Glasgow portion has six sleeping cars, although it does not always load well.
The current system of Highlander (INV/FTW/ABD) in the 21.15 path and Lowlander (GLC/EDB) in the 23.53 path makes perfect sense.