So all all these scenic lines and also the Cumbrian Coast, could 159 three car sets be used, or 2 x 158s, or a 159+158, thus giving longer trains with more seats?
Settle - Carlisle services typically are formed of 2 or 4 carriage class 158 formations (and 3-car trains formed of a 2-car class 158 strengthened with a 153), and within the next couple of years West Highland services will begin to be worked by 2, 4 and, assuming that workings are as I remember them, 6 carriage formations (as far as Crianlarich) of refurbished class 158 units.
Services currently worked by class 158 units on the Kyle and Far North lines are expected to remain as 2-car formations as far as I am aware: It's a shame that Scotrail's class 158 fleet does not seem large enough to permit their refurbished 'scenic trains' to operate as three carriage 'hybrid' units on all Highland routes in the future (As was done with the West Highland 156s for a couple of summers in the early - mid nineties).
It is to be hoped that the arrival of Northern's class 195 fleet might permit their large class 158 fleet to be used to further strengthen remaining class 158 services such as those on the S&C, and the use of class 158 units on the Cumbrian Coast, again preferably in 3 or 4 carriage formation, would certainly be welcome. As you note, if the South West franchise winner chooses to procure new diesel stock, the release of a large number of well maintained class 158 and 159 units, the latter all 3-car trains, could be a game changer in terms of capacity on other provincial routes around Britain.