Dear Chalfont, The Banffshire Journal around 2nd June 1926 looks a good bet for where she is buried. If I were in past the Aberdeen Family History Society, 164 King Street, Aberdeen I would look it up for you but that could be Feb or March time frame. The tapes are on the LHS just past the reception desk so that would be easy.
I see Nethermills on the map just south of Cornhill but no Leathermills.
The Cemetry at Fordyce is a logical choice from Cornhill but your picture kind of rules it out or is very deceptive.
If the Hospital was in Banff as somebody else has posted then the Cemetry could be somewhere outside Banff. It does NOT look like the Cemetry at the School in Banff as it slopes away to the South from where I have seen it and is a busy place. It is NOT Alvah (more relatives in there!).
I have tried the MIs at
http://anesfhs.org.uk/ but no Mary H Stevenson to match your criteria.
Sorry if it sounds "negative" as we rule out the less possible but the "correct result" will come if you persist with it. It was in June 1926 so it can't be that difficult!
Keep at it, nw_whiskers.