Railmiles mileage engine gives the route via Edinburgh from Queen Street as 177 miles 51 chains, compared to 154 miles 45 chains via Stirling and Perth (direct trains)
Disclaimer:I know very little about the technicalities of fare structures compared to more the experienced posters here.
However, I have booked and travelled an anti-clockwise 'scenic route' to Inverness several times viz
from GLQ via ABD to INV
returning INV via EDB to GLQ,
and its has up to now been charged at the same rate as a direct journey from GLQ to INV (to my surprise).
The route has raised eyebrows when tickets were checked during the journey, but the seat reservations seem to have been deemed to have confirmed the route.
For those interested in seeing what happens where giant hogweed infestations are left untreated, I can recommend this route. I will not be surprised to hear of a DMU attacked by tiffids en route from ABD to INV some day soon.