It really doesn't make sense at all. Anyone who wants to do that can already take the Lowlander to Glasgow and a bus (or day train) to FW. There's also a bus from Inverness to FW, though to my great surprise it's nowhere near as scenic as the one to Glasgow.
If you want to simplify the operation, what would make sense is to ditch the Aberdeen and run it as a simple pair of half-trains, one to Inverness and one to FW. Or if you want to keep it, switch round which one you have to swap coaches for recognising that these days demand to FW is higher.
Also...isn't it the case that the seated coaches contain some sort of important electronics, so you can't run a portion without one? As building independent vehicles as per the RIC standards was presumably too sensible?