With the InterRail Great Britain Pass, can you use it to travel on the Night Riviera and Caledonian Sleeper trains in the seated accommodation?
You sure can! You'll require a seat reservation to travel though, and your ticket must be valid for both of the days.
These can be made in advance for free at a Staffed Station upon production of your ticket. The Caledonian Sleeper can also be reserved for free through ScotRail telesales (and I would advise ensuring you reserve in advance).
As a quick note, depending on which Caledonian Sleeper you travel on, it may be necessary to change carriages during the journey (I know the Fort William portion going north requires this at ca. 4am).